Fediverse is a network where servers send user posts to each other, in the effect every server should have the same or similar content. It's not one app, it's a protocol and you can make implementation yourself.
AFAIK Lemmy and Mastodon are connected with each other, but using them to read other one content is kind of buggy/hard. Kbin is easiest to operate if you want to see content of both.
Add also randomization of the instance, in case the main one is down and for privacy. There's already libredirect addon which does that, but still cool.
Depends on your usage, if you use only flatpak, fedora silverblue would be perfect. If you want to manage multiple computers or you're customizing it, switch to nixos and you won't look back.
But both are different than traditional distros, especially nix. If you already are advanced user of linux, you'd have problems with anything more than installing apps on nix for first time.
I doubt many people will move. Most people will just start using the official app or move to another website. Lemmy doesn't have much content (yet), and that's yet worse than not having a good app. They will just quick look and go back, if even.
Theoretically there's "no hate speech" in the rules, but I haven't seen anyone getting banned for that. You'd need to find out yourself what's their definition of this word.
What's the difference between this and the site shortcut?