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Not entirely sure what you're asking here, but do you mean that the chisel should be of the same metal type, or better, when used in the recipe?
If so, you'll have to, unfortunately, make specific recipes for each metal type, and add a list of valid metals for the chisel via "allowedVariants".
"C": { type: "item", code: "game:chisel-*", isTool: true, toolDurabilityCost:10, allowedVariants: ["copper","tinbronze","iron"],  

It looks like the whole shape for the bow is missing. Try adding "game:" prefix to all the shape references:
 

Looks good to me 🙂👍

Might be a limit for how many layers there can be, I didn't look that much into the code for chunk layer generation. Try reducing your layers to below 32, since that seem to be the amount that do get generated.

Not sure where the misconception comes from, but the nests do NOT spawn the locusts!
However, among the nests you will find 1-2 sometimes 3, cage-like cubes, and those are the actual spawners.
They can be among a group of nests, or all on their own, often in the ceiling somewhere, and they have a set limit of locusts they spawn, before needing time to fill up for another "batch" of locusts ready to be spawned.
Agitation of this cage, eg. trying to destroy it, will activate spawning and dump the amount of locusts it has ready. Player proximity will also trigger spawning, but not as rapid as trying to destroy it.
 
Good luck on your endeavours in the depth!

Hi and welcome to the forum!
First off, jumping on a release candidate for a long time world may not be the best choice, as it's usually not recommended for anything else than testing. And going back to 1.20.12 will not be a pleasant experience, as there are significant differences between the two versions, both in assets and code. Upgrading is supported, but downgrading is not as far as I know.
Second, hares only eat from small troughs, as stated in the release notes for 1.21.0-rc1:
Tweak: Only foxes, raccoons, chickens, and hares can eat out of the small trough, while all other larger animals can only eat out of the large trough  
Regarding carburization to make steel, that process takes up to 5 in game days, and have to be refuelled several times to keep the process going. Don't worry if it goes out, the timer will continue where it left off until the steel is done. Once done, the tool tip on the coffin will tell you it's ready and can be broken to get the blistersteel ingots. 

Hi and welcome to the forums!
Sadly, the default VS servers do not support mods, as was informed on the product page before purchase.

Wish I could give you a more joyful reply..

Hi and welcome to the forums!
Yes, you can change where the game saves its data, by using startup parameters.
Client Startup Parameters
The ones you're interested in, will be --dataPath and --addModPath
 
These can be entered into the Target: field in a shortcut, like this:

 
Hope that helps 🙂

"/weather setprecip 1"
This will give you the heavies rainfall the game has to offer
To add to this, you could do "/weather setev heavythunder" to add a thunderstorm
Can add hail with "/weather setev smallhail"
Wiki: Weather
 

Not entirely sure what you're asking, but these might be helpful:
Server startup parameters
Server Config

Jump in and learn as you go, that's what I did when I got the game. I played a good 5-6 months with no mods, just to learn the game and all its mechanics, before jumping on a multiplayer server with lots of mods. There's no right or wrong way to play this game, we all learn in different ways. Just have fun and goof around, explore the world as you go and use the handbook as much as possible for things you're stuck on. Adjusting settings for food spoilage and such, is a personal preference, and do what ever you feel comfortable with. I wish you the best of luck, and welcome to the forums, fellow Vintarian 🫡

/worldconfig clutterObtainable yes
Restart and the config will be effective.
https://wiki.vintagestory.at/index.php/World_Configuration

Yes, any structure/solid surface above the greenhouse, even if it's 100 blocks up, and that is directly above the greenhouse, will block the sunlight and thus the effect. I've had to move my greenhouses several times because I built something on a higher elevation that stretched over the greenhouse.

Try placing some kind of light sources down there, like some oil lamps, and that should limit the spawns a bit.

The yellow "clouds" are highlighting mycelium chunks, alas where mushrooms grows/might grow.
You can turn this on/off with shift + M if I remember correct. If not, the hotkey can be found in your settings, under the "Control" tab.

I would try to adjust the texture atlas size to see if that's the problem. I had a similar problem in my game, and adjusting the texture atlas size fixed it for me. 
Go to %APPDATA%\Roaming\VintagestoryData and open the clientsettings.json file.
In that file search for "maxTextureAtlasWidth" and "maxTextureAtlasHeight", and check what size it's currently set to. Usually 2048 in both.
Try doubling the number in both and check if that helps. I'm using 4096 for my game with lot of mods, and have had no texture issues after I changed this setting.

You could use the paintbrush from QP Chiseltools to help with that. Great set of tools to use, even in creative mode.

Have you tried /whenwillitstopraining command to check what that tell you?

You can turn off the temporal stability by typing /worldconfig temporalStability false to get rid of that effect.

What do you mean by travelling large distances? 200 blocks? 500? More? There can be large areas that are unstable, that's just normal. Likewise, there are large areas that are very stable, some almost all the way down to bedrock (I've had worlds where that has happened). Just try going further than you have been, and see if things change, you've probably just been unlucky on your current world 🙂

The default location is %APPDATA%/VintageStoryData/Saves