LadyWYT

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.

First off, welcome to the forums!
I've heard that boss health is supposed to scale in comparison to nearby player count, but I'm not 100% sure that it does. Overall, I would say that story locations in general are a little easier with a friend or two, simply because you can watch each other's backs or revive each other in the event someone falls(thanks new healing mechanics!). Very minor spoilers in that so far there is one boss fight per chapter, but not every story location contains a boss battle.
 
Each story chapter has at least one "challenge" portion that will prove quite difficult on average, though how difficult it actually is depends heavily on individual player skill. I highly recommend taking temporal gears with you to reset your spawn, in the event that you meet with misfortune.
 
Again, it depends on player skill. On average, the main puzzle locations take a couple of in-game days to complete, minimum. If you've never done those locations before though, I would prepare to spend more time there overall, as some puzzles can be quite challenging.
 
The answer to this one really depends on whether you prefer to complete content solo, or with friends. Completing it solo, you can take as much time as you'd like at each location, and won't need to worry about sharing loot. If you decide to complete it with friends though, hilarious hijinks can ensue, and overall your reactions can play off each other.

Heh heh heh...this is a certain story location from chapter two, and will appear in all worlds that have been updated to 1.20 or later, assuming that lore content is enabled. If you are not currently pursuing the main story, you can venture closer to take a peek at what it is, but I don't recommend venturing into the location itself until you actually have the mission to go there.
Spoilers:
 

Welcome to the forums! 
My initial questions are which mods, and does the issue still happen in the vanilla game? Not saying that a mod is responsible, but when playing a modded client mods are generally the first thing to check when issues arise.
Otherwise, I would suspect that you might be playing with VSync turned on. There were changes to the rendering system in 1.21, I believe, that seem to be more noticeable on certain hardware with VSync enabled(ie, produces a graph like the one in the video). Turning off VSync may smooth out the graph, however, I would also note that your framerate still seems to remain high despite the issues. Unless your frames are dropping enough to make the game actively choppy to play, I'm not sure that I would worry too much about it, in the event it is VSync related or something.

It used to be a thing, but was phased out in 1.20 I believe. You'd have to mod it back into the game.

To my knowledge they can mature, however, unless you're intending to keep them as curiosities, there's not really a point to keeping them around. They can't be domesticated, unless you're using mods to do so, and the resources they offer aren't very good in comparison to other animals, especially proper livestock. 

Welcome to the forums! I am pretty sure that if you placed torches at point A and point B, you'd get lighting like the very bottom strip. That is, both ends are the brightest spots, with the light getting dimmer towards the middle.

They are, but unfortunately, that is likely the cause of the problem. Herbarium received an update recently, but I'm not sure that the issue is fully fixed. The Wildcraft mods themselves aren't yet updated for 1.21, save for the trees module, I think. There is an unofficial mod that makes the Wildcraft mods function on 1.21, but that's really about all it does...makes the mods run. It doesn't really fix any underlying issues that might exist.
In any case, you can try checking for any updates of the mods you're using, and seeing if installing any applicable updates fixes the problem. Otherwise, I would recommend starting over and leaving out the mods that are causing problems.

Welcome to the forums! If you change world generation settings, the new settings will apply to newly discovered chunk, but not the old. You would need to regenerate old chunks entirely for the changes to take effect. 
To regenerate chunks, I think the command is /wgen regen [chunk radius]; I do not recommend doing this in chunks that you've built in, or chunks that otherwise have things you wish to keep. If you go this route, I also recommend keeping the chunk radius value small, as the more chunks you try to regen at once the longer it will take.
Of course, if you aren't particularly attached to your current world, there's always the option of starting anew with the correct settings. That saves you the hassle of trying to fix things via commands.

Nah, you can just wait until the pile burns out before refilling--it won't hurt anything. The process will just remain paused until you refill and reignite the fuel piles.

If you aren't having any luck in that area, I would try somewhere else. Ideally, you want a High or better reading, but it also doesn't hurt to try digging at Poor or Very Poor readings. Since iron deposits are so large, they're generally hard to miss when digging a mineshaft, so if you dug a shaft and found nothing best to try elsewhere.