Diff

Go about downgrading? Go the client download area and click "(Show all available downloads and mirrors of Vintage Story)" to show older versions. Go about backing up worlds? They're stored in your VintagestoryData folder, copy them somewhere safe, and copy them back if you need to revert. Loading a world in a newer version of the game will update it and can make it incompatible with older versions, so backing up is an important step if you might want to go backwards.
Code mods are all incompatible between 1.21 and 1.22 so you'll get crashes unless you make sure you purge any that don't belong with whatever version you're currently running. It can be a bit of a mess of things to manage, so multiple parallel installations like dakko said is probably a smoother way to test.

Edit the existing release and it'll let you re-select the versions your mod is compatible with

Did you copy the clientsettings.json file? It contains the directory mods load from, so if you don't update that for Linux it'll be looking for a Windows path. Shot myself in the foot with that one a while back. Like you saw, the button still opens the Linux path ignoring what's in clientsettings.json which makes it even more confusing.

Easier way, type just 
sudo xattr -rd com.apple.quarantine (make sure to include an extra space after "com.apple.quarantine ")
and then drag and drop the Vintage Story app into the terminal and hit enter.

Restart often fixes that on Mint

Published to ModDB: https://mods.vintagestory.at/radialhotgear

Far as I can tell there's nothing wrong with the shape of your pit, so it's either that there's a stack that's not quite a full 32 logs, or that it's not being lit correctly. I recreated your pit and was able to get it to bake down to crispy charcoal bits. Things to watch for, once I complete the fire pit on top of the logs, the info HUD at the top actually changes to "Charcoal Pit" instead of "Fire Pit", and after lighting it, it displays a warning that you have 30 in game minutes to cover the pit. If you don't see any of those things, abandon ship and double check everything.
https://www.youtube.com/watch?v=vLHjLE9Y-Oc
 

Looks like a complex answer, this is the code responsible, but it varies based on a lot of things and I'm not finding a single max speed written in stone. You'd have to trace out how high the math goes to figure out the max speed.
I also can't find any multipliers for boat type. I believe this code handles both sailboats and rafts, but the only modifiers I see for speed are the status of the sail and how many people are rowing.

The server is running, so you need to stop it. Either restart your computer, or open Mint's equivalent of the task manager to kill the server. Or "killall Vintage" in a terminal would probably do the trick.

Sounds like your Linux experimentation is going pretty well so far, that's great to see!
Now to be a killjoy, I really wouldn't recommend starting it with sudo. If a hacker discovers a vulnerability in the VS server, you could be hosing your entire system. Instead, since the server's telling you which directory it wants, I'd recommend doing something like
sudo mkdir /var/vintagestory/ sudo chown vintagestoryuseryoucreated /var/vintagestory Which will create the directory and give it wholly over to the vintage story user you created so it can do whatever it needs to with it.
On Linux, if you haven't already run into this info yet, files and folders can be owned by a user and assigned to a user group, and permissions are split into groups of three. There are read, write, and execute permissions, and those can be given to the owner of the file, given to the group associated with the file, and blanket permissions for everyone else. The chown command changes the owner of a file or folder, and if you needed to edit permissions, you can use chmod like this:
sudo chmod u+rwx /path/to/file sudo chmod g-wx /path/to/file sudo chmod o-rwx /path/to/file Which gives the user (owner) of the file all permissions, removes write and execute permissions from the associated user group, and removes all permissions from all others.
In general on Linux, you'll typically want to avoid running things with sudo, particularly software that will stay running for a long time. It often works, but it takes absolutely all the safeties off. Many services will outright shut themselves down and refuse to run if they notice they're running as the root user.

Crossplay works great! With the magic of Microsoft you don't even need to cross compile (nearly all) mods. I develop mods on my MacBook that I then run on my Steam Deck, playing multiplayer on (probably) Linux based servers with other players who are running Windows clients.

I've seen only one mod that isn't cross platform, and it was a mod that overrides the font the game uses by using a Windows-specific system call.

After doing way too much to work around this myself, I discovered this:
https://wiki.vintagestory.at/Installing_the_game_on_Linux#Easy_installation_options

It was always a requirement, but in 1.21 it did change from .net 7 to .net 8.

Flathub hasn't been updated with 1.21 yet. It's not fully automatic, it needs manual approval from the maintainer, so you can't do it through Discover until it flushes through. It's on its way:
https://github.com/flathub/at.vintagestory.VintageStory/pull/101
But when testing it, it crashed for me. It looks like it needs .NET 8.0 installed, but the Flatpak is only set up to ask for 7.0. I think it's probably best to wait for the moment.