Maestro

Yes, the instance you signed up on would be the identity provider

That’s a solved problem from a technical perspective. Use OAuth. Just look at “sign in with google/facebook/github/etc”

You could try fedia.io if you prefer the Kbin/Mbin interface over Lemmy

I’m not sure. Similar communities at different instances can have very different rules and vibes. There’s a reason people prefer talking politics on Beehaw versus Hexbear.

I like the Kbin solution so far. Leave communities separate but cross-link and deduplicate individual threads from multiple communities in your feed. The implementation at Kbin is still a bit flawed, but the idea is sound.

Maybe. I don’t think anyone’s thought that far ahead yet. At least the lemmy devs give hosters the option to add a click through agreement to the signup page.

You can’t rely on fair use because fair use works differently around the world, if it exists at all.

As for copyright, you agree to terms & conditions when you sign up for your fediverse account. These should include language that grants permission to use everything you submit for federation.

Federation means making copies. Hundreds of them. If you attach a copyright license to a comment then each and every instance that federates your comment would have to abide by it. That is just not possible. If you want to write stuff on the internet and copyright it, start a blog or something. By posting on the fediverse you implicitly allow copying.