Streetwind

Rift activity is unrelated to temporal stability. It's just about how many drifters are likely to spawn at any given time. Low rift activity = few spawns; high activity = you'll get mobbed. It'll change over time and is not bound to any given area.
Only the spinning cogwheel in the center of your hotbar tells you whether a region is stable or not. Counterclockwise = unstable, clockwise or holding still at maximum fill state = stable. Temporal stability is bound to the local area and never changes over time.
 

0.5%, according to ...\assets\survival\blocktypes\stone\rock.json.

It does not - only a server restart is required.
 
This, however, very well might. World size and height parameters are not something you're supposed to change after starting a map. Trying to find ways to do it anyway is liable to result in a broken save.
Tweaking the passage of time is also not without dangers. The game explicitly forbids going back in time, meaning that while you can jump forwards to a later date, or accelerate the flow of time, the only answer to "oops I went to far, how do I go back?" is "you don't". Again, trying to find ways around that is (not just pretty much impossible but also) likely to ruin the world you try it in.
There are also quite a number of commands that modify the flow of time, and you need to know precisely what you want to achieve in order to choose the right one. Otherwise, a different command might still give you what you want, but come with other effects you didn't want.
 
So in both cases, I recommend the three of you play around for a bit in a standard world, if only to figure out what you would like changed. It's hard for us to recommend you any particular settings without you specifying at least some part of what you're looking for, because it often depends a lot on your server. For example, large servers that have players on them 24/7 tend to have problems with seasons rushing by and food spoiling while players are offline, so I'd recommand to such a server that they increase their days per month setting and slow down food spoiling speed. On the other hand, small servers do not have this problem, because servers automatically stop time if no one is online. If the three of you only ever log in together, or if your server is actually a singleplayer world shared for public access which will turn off as soon as the host stops playing, then you won't have these issues and don't need to follow the above recommendation.

The display is wrong. It's wrong on all the special trees. You may need as high as bronze, but I think copper works on some of them too.

Unfortunately this is not possible. VS allows you to do a lot of different things to the ingame time, through different commands, but the one thing you are never allowed to do is go backwards in time. In fact, the dev team is very quick to patch out any loophole that surfaces which lets you do so.
The reason for this is simple: reversing time tends to corrupt your save. A lot. Often, that corruption is irreversible, leading to a total loss of the whole savegame (unless you had a backup).
And honestly, why is it so important which year you are in? You wanted to go back to spring, and the command made it happen. That the date is slightly different doesn't affect gameplay in any way.

There's no such thing as upgrading a purchase to a different one in the shop. You could try writing a support ticket and ask the team to accomodate you on goodwill, but it probably won't work.
However, yes, you can use the same game account on any number of installations. Just keep in mind that your savegames and mods don't magically jump computers with you. They're stored locally on each machine.

Stop right there criminal scum. That's illegal! The realism police has decreed it!

This should have the answer you're looking for:
 

You will not be able to grow the same type of crop on the same block of farmland over and over.
Each crop requires one of the three nutrients, and drains it from the soil. The less nutrient remains, the slower the crop grows. Nutrient recovers over time, but much slower than plants consume it. Fertilizer helps, but releases only gradually over time (hence the tooltip literally saying "slow-release fertilizer"). Some crops with high nutrient needs will still drain the soil much faster than fertilizer can restore it.
To get around this, you need crop rotation. That means: first, plant a K crop. Once its fully grown and harvested, plant a N crop. Once that's fully grown and harvested, plant a P crop. And once that is done, the K nutrient will have fully recovered and you can start again from the beginning. You don't even need fertilizer or good soil for this to work. If you do have good soil and fertilizer, you may be able to alternate between just two crop types instead of all three.
But you can't expect to just plant flax over and over without a break on the same soil. This is intentional.

Pretty sure this is hardcoded.

Less valuable items will typically be traded in multiples.
That is, he may show "8 feathers", and have a demand of "2", meaning he's buying two stacks of eight feathers each. Since you only offer two feathers, you're not even offering one stack, so there can be no trade.
Therefore, doublecheck the stack size of the offer.

I think the location is unique. So just one per world.

Flax has a temperature tolerance of -5°C. The greenhouse makes the plant treat the ambient temperature as 5°C higher, so effectively it can go to -10°C now.
Your ambient temperature dropped below -10°C at some point, and the plant got cold damaged.
Once the plant gets damaged, it doesn't recover.

When installing mods on an existing save,
- some mods simply just work, and
- other mods will not work fully because they rely on world/terrain generation to enable their content, but
- no mods will cause problems with your save
When removing mods from an existing save,
- a few will be possible to remove without a trace,
- a few will cause your world to no longer work, and
- most will cause a graceful failure state where they can be removed just fine but leave behind invalid blocks with missing textures

Yes, the calendar pauses while no one is online.

From 1.17.7 forwards, the game will pause when the player opens the handbook in singleplayer mode. This was a much and long requested improvement among the playerbase. (Obviously no change in behavior in multiplayer).
Perhaps your nephew used to open the handbook to let the world generate? But no, that wouldn't have been a true and proper pause in old versions, meaning that time would pass, the character would grow hungry, and enemies could attack the player. So it can't have been that.
The choose-your-character screen was also upgraded from simply freezing the ingame time progression (which still left you vulnerable to attacks and hunger) to a true pause in one of the more recent versions. Can't for the life of me remember which one though.
I just tested in 1.15.9, which is the oldest version I still have lying around. Even back then, a true pause did not allow world generation to proceed in the background; everything is completely stopped. So whatever your nephew used, it was never a true pause. Just something that felt like it. Probably the character creation screen.
 

Right. Regarding fruit trees, the most temperature-sensitive of the tropics trees start having a chance to die once the temperature drops to below +10°C. I don't think it happens immediately upon reaching that temperature; it's probably more something like being below that temperature for a full day. Also, it is possible for the tree to survive down to +6°C, although the chance grows smaller the colder it gets.
Meanwhile, the non-tropics trees need vernalization, and for that to trigger, multiple full days need to pass without the temperature ever climbing above +1.5°C at any point. There's a chance it might still work up to +2.5°C, but for guaranteed fruiting you want a degree below that. And remember, it must not rise above that temp at any point, so that would limit the daily maximum only; daily minimum during the same time will probably be -15°C or even lower. And that's a big problem for the tropics trees, as you can imagine.
Placing a fruit tree into a greenhouse will allow it to vernalize more easily. The +5°C greenhouse bonus is applied to vernalization temperature. It is not, however, applied to any other temperature governing fruit trees.
Peach trees have a die-below temperature of only -12°C, but still need vernalisation, so those are the hardest to cultivate out of all trees. The climate where that works out is a much narrower band than for the other trees. But greenhouses can help here.
All other vernalizing trees can go much colder, and you basically don't need to worry about their survival at all unless you move northwards from spawn. But since you want to accomodate tropics trees, you'll be moving south, and trying to find an area where tropics trees can thrive at sea level while planting vernalizing trees far up a mountain.
 

Each bloomery does 120 nuggets (5 blooms) at a time, yeah.
The solution is moar bloomeries. I typically do two or three in parallel when I work iron, but I only smelt some when I need it. I have never tried to smelt a whole iron vein all at once - I value my sanity Processing those blooms without at least four helvehammers would take ages, and I can't be bothered to build that many when one or two will cover my actual needs.
But if I tried to do mass-smelting... Well, I'd crush all my ore into nuggets, and then count the number of slots taken up by those nuggets. 1 slot holds 128, a bloomery holds 120, so it's a good first order approximation to build one bloomery per slot. Whatever's left over can go into a second run. It'll likely be done well before you get the blooms from the first run processed.
One bloomery happens to take 12 fire clay bricks, and you fire clay bricks in sets of 12... so you just fill one pit kiln with bricks for every bloomery you want to build.
You also get like 8-10 bricks back when you break a bloomery, so you can use those in a second run for whatever ore you'll have left over.

Quoting from the patch notes:
What exactly hasn't been working for you?

Known issue with 1.18.5. Fix is pending for 1.18.6.

For the lower tier enemies, using a shield and just taking the blow is great. Neither surface drifters nor deep drifters nor bronze locusts deal enough damage to overcome a raised shield, so you just watch them flail ineffectually and laugh as you murder them. Even tainted drifters only deal minor damage.
For stronger enemies - sawblade locusts, bears, nightmare drifters - you want to dodge and kite. They have too much HP to kill them quickly, deal a lot of damage even past a shield, and can often shred armor at accelerated rates. In that case, you have to time your strikes just right, so that the moment where the hit happens you are in range and then immediately retreat. You can use a bronze spear in melee to make this easier, because the spear has a longer range, so you have more safety margin; but it also does a lot less damage than a high-tier falx, so it's not ideal either. Ideally you'd bring a good bow, or even a sling with several stacks of stones.

Hostile creature spawning in VS works a lot like in Minecraft: wherever the light level is low enough, drifters can spawn.
When rifts were introduced, VS added an additional rule on top: drifter spawning is forbidden under the open sky. Only places that have a solid block above them qualify for spawning (given appropriately low light level). Rifts break this rule, allowing drifters to spawn under the open sky in a small radius around them.
When you turn off rifts, you simply turn off this extra rule and revert the game to Minecraft behavior: drifters can spawn anywhere the light level is low enough, even under the open sky. There is no setting that turns off hostile creature spawning.
However, with creatureHostility=off, these creatures should not attack you.
Additionally, there may be mods available that remove hostile creatures entirely. Click the "Mods" button in the navigation bar at the top of the forums to go to the official mod database.
 

This happens when the game saves just as you exit the bed. That stops the time acceleration from smoothly winding down.
To fix this, get back into bed (so that time starts accelerating faster again) and then exit it (so it stops accelerating).

This is new with 1.18, in a way, though not as new as you may think.
Food spoil speed has always scaled with temperature. The hotter it gets - and I notice in your screenshot that it is summer - the more quickly perishables will spoil. In previous versions, storage containers like clay vessels would display only their own modifier to food spoil speed, while the modifier from temperature was applied silently in the background and was not shown anywhere. From 1.18 onwards, storage containers show the sum of all modifiers applying to food stored within.

/worldconfig foodSpoilSpeed 0
Although I'm not sure if 0 is a valid value here. Some of these worldconfigs have minimum lower limits, you'll have to experiment. The closer to 0 you can set it, the slower food will spoil.
After you enter this command, exit to main menu and reload your world, then enter the command again without a number to let it print the currently active value. If you entered something invalid (for example if it doesn't like 0) it will likely display the default value of 1. Then you can try something else, like 0.1.