The fdroid repos can get kinda outdated. But in my personal experience, the app has shown no issues. I guess being compliant with the tirrent protocols is all that matters for it to work.
There isnt much to say about how to use it. Just install the app, search for a torrent you want to download (be it via a magnet link, or a .torrent file) and thats it.
To answer questions 1, 2, and 3:
Yes you can 👍
yep, do your rice. Maybe visit the Unixporn community.
For the most part (in MY experience) changing DEs only involves installing the desktop package, choosing your login manager, and then removing the old DE and its native apps (unless you want to keep them)