LadyWYT

Honestly, with it being a very old world, I would submit it to the issue tracker. 
If you can manage to repair the chunks even a little at a time(it sounds like you've managed to do this?), an alternate solution is to repair only the chunks you care most about, save the builds as blueprints(or however this happens via worldedit), and then import the builds into a brand new world. For items, make a list of the important stuff on paper, and then use creative to give it all back.
Unfortunately, it's not a solution that preserves the world, but worst case scenario, it should allow you to at least preserve the builds.

Welcome to the forums! You basically have two options here, the first of which is just doing a LOT more prospecting. From the looks of the map here, you've done a lot of prospecting in a few chunks very close to home, but little more. I would suggest doing more general searches, in a 1000 block radius(more or less) from your base. Once you pick up another trace of cassiterite, then you'll want to do more localized density searches in that area to see where the best dig spot is.
The second option is to just dig at the best reading you've already found, and see if there's any ore. Poor and Very Poor readings aren't ideal for digging, but that doesn't mean that the ore in question failed to spawn, just that it's not too likely to have spawned there. Cassiterite is a common ore, so it's generally not too hard to find, even at less than ideal readings.

Welcome to the forums(and the game)!
Ultimately, the choice is up to you, but since you've not even played the game yet, I highly recommend playing just vanilla for a while and just focus on figuring out the basics first. Doing this will give you a much better grasp of the game overall, as well as allow you time to figure out which parts of the base game you'd most like to change.
Regarding vanilla options, I would recommend picking Standard or Exploration as the starting difficulty. Standard is tough, but fair, and also the intended developer experience. Exploration is less challenging, but may be the more attractive option if you find yourself struggling too much while trying to learn. Wilderness Survival and Homo Sapiens are both the hardest difficulties, and while it is possible to start learning the game on these difficulties, it's definitely not something I'd recommend for brand new players unless they really really want that kind of challenge.
One important thing to note about vanilla options, is that most of them can be changed via console commands after creating the world(though you may need to reload the world for the changes to take effect). A couple of options you might consider tweaking, are enabling the Color-Accurate World Map under the Survival Challenges tab, as well as setting a brief grace period for monster spawns under the Player Spawn/Death tab(in this case, rifts will still appear if enabled, but won't spawn monsters until the grace timer is up). You may also wish to enable the "keep inventory" option for Death Punishment, if you find yourself dying a little too often.

Welcome to the forums! Unfortunately, I'm not sure that there's currently a way to adjust the stability aside from turning off the mechanic entirely. The best fix is to just move your smithing setup to a more stable area, or else be prepared to burn a lot of temporal gears and healing items to keep your stability topped off.
If you're willing to mod, you could try one of these, though bear in mind that mods aren't guaranteed to be maintained, and may have unintended consequences. Older mods will sometimes work on a newer version, but best to make a backup of your world just in case something goes wrong.
Temporal Amulets: https://mods.vintagestory.at/temporalamulets
Temporal StabiliTea: https://mods.vintagestory.at/show/mod/27496
Chunk Stability Override: https://mods.vintagestory.at/chunkstaboverride

Yeah, if you just press and hold shift(or whichever key you have sneak/crouch bound to), your character should climb down the ladder without issue. If you're falling off the ladder while trying to do so, it's likely because you are pressing the backward movement key while descending(or perhaps pressing one of the side movement keys by accident) which will cause your character to move off the ladder and thus fall.

You'll want to be looking in taiga areas for them, as they prefer cooler climates. So searching the cool climate zones is a good place to start. They can spawn in the temperate zones as well, though on high altitude plateaus(typically grassland or sparse forest) where it's cool enough for a suitable habitat. Those would be the prime spots to look.
That being said, I wouldn't necessarily call them a common spawn, as I've only seen them in the wild a handful of times. It's probably easier to find them, than it is to find a baby one that you can actually capture and tame, since I'm not sure that deer can be bred like normal livestock species can be. Unless you're playing Homo Sapiens, or otherwise just really wanting to catch and raise your own, it's much easier to purchase a tamed adult elk from your local treasure hunter.

Okay, after taking a quick gander, you should be able to work your way out of this without using the console. First, make sure you're at full health, then go stand on the ledge outside the doorway. Facing the doorway, step backwards off the ledge and immediately press the forward movement key to hug the wall directly in front of you. It's a decent drop, but you should survive. Heal back up, then work your way down to the bottom of the structure carefully--the drops should be much less drastic from here. Once at the bottom, take a look around and be on your guard for monsters, as there's probably a few lurking around here. Somewhere at the bottom, there should be a ladder to climb, that will lead you to the path out. Once you find the ladder, simply climb it and work your way back to the upper levels.
Of course, the easier, safer option is to just switch to creative(/gm 2), fly back up to wooden platform, and then switch back to survival(/gm 1).

Yes, that is what you need to do--trap an elk calf, and then feed it enough before it grows up in order to have a partially tamed elk. From there, you can saddlebreak it to achieve full tameness.
 
You can try searching mountaintops or other high altitude places, as those may be cool enough to spawn elk that far south. Otherwise, your best bet is making a trek north...way north...and establishing a temporary base for a year or two while conducting your search. Elk are most common in cool taiga regions, typically grasslands or sparse forest(I've not seen them in dense forest--they may still spawn there but will be difficult to see). In temperate zones, this typically means poking around on high altitude plateaus.
Of course, finding the elk is probably the easiest part. It's acquiring an elk calf that's going to be difficult, since I'm not sure that you can actually breed deer to get babies.

Like @Maelstrom said, rainfall doesn't affect season length or outside temperature. What affects those is latitude and altitude.
Body temperature, however, is a different story. If it's raining and cold enough outside and you get wet, you'll get cold a lot faster than if you were otherwise dry. 

To my knowledge, bosses will ignore players who are in creative mode.
 
Try switching to survival mode; the Crow should show up, though it may take a minute or two depending on where exactly the Crow is at the time. Ordinarily, the creature patrols the waste and will circle the Tower once the player reaches that area and begins the climb. However, if you're in survival mode and it's been several minutes, it's also possible that the boss is bugged(this issue has cropped up a couple of times over the patches), in which case you should head over to the github bug tracker and report it.

To my knowledge, it only affects crops and other things gathered in the wild, and not crops cultivated yourself.

I don't think predators will go after tamed elk, but it's better to be safe than sorry and try to leave your elk somewhere relatively safe when not riding him. If tragedy should strike, the good news is that elk will revert to a mortally wounded state and you will have 24 in-game hours to revive them with a bandage before they actually die. Of course if they are injured, you can also heal them immediately if you have a bandage to do so--no need to wait for the mortally wounded state! And if you fall into a cave, the easiest way to get the elk out is to dismount, climb out, and call the elk with a bone flute. The elk should teleport to you, if no pathing is found otherwise, but the bone flute also requires you to have claimed the elk with a medallion(any will do) in order to work.

Actually, older worlds should be able to update to 1.21 just fine, assuming they were not worlds created on a release candidate. The devs have been pretty careful about doing what they can to make sure older worlds can remain viable on newer game versions.
 
Those should survive an update, yes. 
 
Mods are going to be the main reason a world might not be able to be updated, so for the best chances of keeping a world across multiple game versions it's best to stick to vanilla. If playing a modded world that's intended to be long term though, it's best to stick to a small list of mods that are known to be kept up to date. The more popular mods like Primitive Survival and whatnot are good choices, as these have been consistently updated(and quickly) for new game versions, and are likely to be kept up to date by others in the community should the original author/caretaker step away.

It should be the VAO option--that's the one I've used while hunting memory leaks. Make sure you check that option every time before loading the world, as it does seem to like to turn itself off.
 
I get it on PC with Dev mode off, so I don't think it's just a Mac thing. I don't recall if there was anything I did to enable it though...I figure it just kinda happens automatically.

As a general rule, the mod versions should be up-to-date with whatever version of the game that the server is running. Older mods will still sometimes work on newer versions of the game, but it's not a guarantee. Mods made for newer game versions most likely won't work on older game versions due to changes in the game's code. You could still try it, and see what happens of course, but don't test on worlds that you care about.

Welcome to the forums! @Thorfinn has covered most of it, in that the handbook covers pretty much all the information you need/could want to know about various game items and systems. It has a Guide section that includes a brief summary of different progression "tiers", and the general steps of how to advance through them. The handbook is also good about cross-referencing things, so there are often links on one page that will take you to other relevant game items and concepts. It's a lot to handle at first, but reading through the Guide section and picking simple first goal from there is a good place to start.
I will also note that there is an optional tutorial upon first creating a world, that will walk you through the process of learning basic controls and acquiring your first set of handbaskets and starter tools. It's not much, but if you haven't played the game in quite some time it could be a handy refresher.
Overall, I would say that the general vibe of VS is that the player needs to learn to think quickly on their feet, as well as get good at planning certain activities well in advance. Death happens a lot, especially for newer players, however, I will note that the game is rarely unfair about it. Most of the time, there was something the player could have done to avoid death, so when it does occur it's important to review the circumstances and figure out how to improve in order to avoid similar situations in the future.
As for in-game hints...there aren't any for standard gameplay, at least if you exclude the handbook. You can learn certain bits about the world through simple observation, but the handbook is still going to be the primary source of information when it comes to learning gameplay systems themselves. In-game hints apply more to the game's main story and lore--a lot of the storytelling is done via various clues hidden in location design and certain flavor texts for certain items, and the player is often left to draw their own conclusions. I will note though, that if a lore item/NPC makes a point of describing something as dangerous, you should absolutely pay attention, as those warnings do actually mean something.

Not really, since the traders will still sell elk. It's only important for the Homo Sapiens gamemode, where purchasing isn't an option at all. On normal gamemodes, the only advantage to capturing and taming wild elk calves is that you won't have to pay the trader for one(though you will have to go through a LOT more work).
 
I think when it comes to creature spawns, it's kind of both. Generally, I play temperate starts, and it's not unusual to only see moose in the wintertime, or at high altitudes; same goes for brown bears. I've even seen the odd polar bear roaming around on mountaintop glaciers outside of the arctic. So I think the creature spawns are tied mostly to the year-round temperatures, as that determines the overall climate of that particular area. In the cases that current temperature might be influencing creature spawns, I think the creature spawns are still limited to what you could realistically expect to find for that climate zone, hence why you might find a moose or two in temperate regions in winter, but won't find polar bears.
As for elk and where to find them...generally it's best to look in cooler climate regions. The arctic is too cold, and temperate is generally too warm, so the cool climate zone is overall the best place to look. For the default temperate start, you'll want to be looking in highland areas, since these areas are cold enough to count as taiga due to the elevation. Grasslands are the easiest places to spot them. If you're seeing spawns like mountain goats, ibex, and larch/pine trees, you're likely at a high enough elevation where you could expect to find elk.
To my knowledge, it's not possible, outside of certain modded creatures, or vanilla creatures that require specific elements present(like forest floor) in order to spawn. I think the best you could hope for here is finding some elk first, and then killing them all if no babies spawned, in the hopes that a baby elk will spawn in later(you could try breeding the adults, but I'm not sure that this is possible currently).
Honestly though, the easiest way to get a tamed elk currently is to just buy one. Raising a baby elk and then saddlebreaking it will save you the gears, but it also requires a significant investment in time and resources, even moreso if the spawn RNG isn't kind.

They will need to sit on the eggs in order to hatch chicks, yes. The hatching timer will be paused while the hen is off the nest, but once she returns the timer will resume ticking. As for whether the chickens will come when you put food in the troughs, it depends on whether or not they're hungry at the time. If they're hungry, they'll try to come eat, but may run away if they aren't very tame and you're standing too close. If the chickens aren't hungry, or aren't able to eat due to being spooked, they'll just visit the troughs once they are hungry and calm enough to eat, provided that the chunks are loaded.

This might be what you're looking for: https://mods.vintagestory.at/chunkstaboverride

You don't want to look for marble as your source of lime, as it's very rare. If you happen to stumble across a deposit you could use it for lime, but you may want to save it for fancy building given that there are better sources of lime to be had.
Limestone and chalk are fairly common finds, and it's not uncommon to find one of the two alongside bauxite(bright orange rock, you will need this later). Depending on what kind of rock you have around spawn though, you may need to do some traveling in order to find it. The rock layers you want to look for are sedimentary, which are on the surface.
Lime can also be acquired via grinding up seashells; it's certainly not efficient, but it'll do in a pinch and you'll get two lime per shell. You can also sometimes find lime/lime-producing rocks in ruins, and can trade for lime at commodities/survival goods traders.
If you're after lime for leatherworking, you should also keep an eye out for borax(found in sediment rocks). Borax is more efficient for leatherworking, and will be needed later for working meteoric iron and creating steel.

Oh gotcha. Yeah spawn radius is what you're looking for. I would try turning that down to like, five blocks or so. You should spawn right next to each other in that case, while still having a bit of wiggle room.
 
Gotcha. I can't really speak for the future plans of the game since I'm not a dev, but there is a roadmap of planned features that you can find here: https://www.vintagestory.at/roadmap.html/
However, this isn't a feature that I would really expect to see, since it's essentially already implemented in the game with the method I mentioned earlier. Essentially, create a singleplayer world, open it to LAN and start playing if you and your friends are on the same network. If not on the same network, you'll still need to open to LAN, then open the pause menu again to open the world to internet. You may also need to adjust firewall rules or router settings for this method to work, and the world will only be playable while whoever is on the host machine is playing.
https://www.vintagestory.at/selfhosting/

To my knowledge, it's mostly RNG. However, it does seem like bears are a bit more inclined to show up in the fall, or at least it seems like I tend to see brown bears more often in the fall and winter than spring and summer. Black bears I see whenever. I've also seen the occasional polar bear spawn on mountaintop glaciers outside the arctic. My best guess is that the temperature parameters might be better meeting the spawn requirements for certain bears at certain times of the year, which is how polar bears can be on glaciers or brown bears seem to show up more often in the fall. 
But that's all pure speculation on my part.
They do not disappear, no. While the game does try to simulate realistic animal behaviors, the simulation isn't that realistic. At least not yet.

The item in question is called something like the elucidatory vessel/gearbox, or something to that effect. Should be simple enough to pop into creative and give yourself one, instead of running all the way back and hoping it hasn't despawned. It's possible that the gearbox hasn't despawned, as dropped items do seem to hang around in unloaded chunks(I've dumped things in the wilderness and run across them much, much later), but it's not really worth a trip back specifically to check for it. However...
 
I think this is the bigger issue, potentially. You mentioned grabbing the glider schematic, but didn't mention the resonator. You'll need to interact with it in order to get the information you need to start chapter two. From the sounds of it though, you're intending to make a trip or two back anyway for books and things, so in that case you can just check for the gearbox then(and give one to yourself if it despawned) and otherwise take care of the resonator, if you haven't already. I do recommend making that second trip sooner rather than later though, as the boss will respawn(one in-game week, I think).

First off, welcome to the forums!
Pressing X will switch the current item to your offhand slot, if applicable--bear in mind that holding something in your offhand is an extra 20% to your hunger rate. Pressing Q will toss an item to the ground, and I think pressing either shift or ctrl with Q(I don't recall which) will toss the whole stack.
 
The default settings mean that the map will be primarily land, with no large bodies of water. However, the 97.5% means that you will eventually find an ocean, if you explore enough. For more ocean though, I would recommend starting over and setting the land cover ratio to something more like 80%--that should ensure a handful of oceans to find, with relative ease, while still having land-based travel predominant.
 
By default, the beginning climate zone is temperate, although the higher up you go the colder it can get. You can turn on coordinates in the Interface tab of the Settings menu--the Y coordinate is what your altitude is. Generally, unless you're building up on the very top of a mountain, altitude isn't something to worry about. If you happen to be on a high altitude plateau, the climate may be more similar to taiga conditions than to temperate proper, but in that case, you should be able to tell by taiga creatures like elk, caribou, or mountain goats spawning nearby(potentially).
The difference between taiga and temperate winters isn't that much. Taiga will be a bit colder and last a bit longer than temperate winters, but it should still be relatively easy to handle with some planning.
Clay is something that can be difficult to spot, until you learn what it looks like. Discolorations can be a prime indicator, if playing with map colors, so you're on the right track there. Clay is most easily found in grassy areas with decent rainfall, or by low altitude lakes and ponds, and it's more easily spotted if the sides of the blocks are visible since they're a different color than dirt.
Red clay is the most common type to find, and can occur at lower and higher altitudes. Blue clay only occurs at low altitudes, typically near lakes and ponds. Fire clay only spawns in bauxite biomes, or underneath black coal/anthracite beds. You can also acquire clay from panning sand/gravel or as cracked vessel loot, but it's better to find a deposit than rely on these methods.
The map should be giving you the Y coordinate value for terrain you've already explored, so this sounds like a potential bug(assuming you aren't playing with mods that could affect such).
 
Honestly, it doesn't sound like the best of spawns, though it should still be manageable for a new player...albeit perhaps more challenging than the average spawn. Generally, the average experience on a temperate start is going to be at lower altitude, with ready access to cattails and berry bushes and other convenient early game resources. I would recommend starting a new world with a spawn you like better, and saving this world for when you have a bit more experience with the game. Of course, if you don't mind the bit of extra challenge, continuing on this world should be fine as well.

Welcome to the forums! To my knowledge, the game should prompt him and download/install the mods he needs automatically with the simple click of a button when he first tries to join the modded world. The only potential issue I could see is performance, depending on which mods you install, since you're running a server and playing on the same PC, essentially.