Could be worth splitting your questions across posts, but given its here now, ill have a crack at the first few questions.

Arch wiki is pretty good, if you need more help, ask here or somewhere similar?

Your desktop environment settings are stored in your home directory, as long as you dont delete them, it should reinstall just fine.

Having home on a separate partition is safe, and a fairly good idea, it lets you wipe your root and reinstall without losing your home data. You can move your home off the root by creating the partition, mounting it somewhere temporarily, copying all files across, and them set it to mount to /home in fstab.

Rust works on windows, so exes are definitely possible. And i think you can cross compile for windows if you want/need to.

Dunno what you mean about crazy, do you mean complexity wise? I have found that rust is really painful to get started, but eventually it'll click, and it starts to make sense.

Git and cargo are two different things, i think its currently recomended that you dont commit the cargo.lock file. https://stackoverflow.com/questions/43667176/what-files-in-a-cargo-project-should-be-in-my-gitignore

Cant think of anything specific for your windows question, try it and ask questions when you get stuck?

I would consider asking questions in more general communities, this one is a little specific, so you may not be seen by the people who can help you.