Selfhosted

If you don't want to moderate, don't let others sign up to your instance. That deals with pretty much all your legal issues.

For US-based people, register for DMCA notifications. This means if someone posts copyright infringing content, you get a message about it instead of your hosting provider, and you get to remove that content instead of your hosting provider removing your account.

Check about GDPR compliance. Part of that is fully deleting user content in a timely fashion when they ask. I've heard that Lemmy might not be good about that, but I'm not sure. If you have any EU users, you'll need to comply.

If you want to run a large instance, you'll need to have a plan regarding CSAM.

If you do moderate, use The Bad Space or shared block lists to defederate from the the most problematic instances. I'm not sure whether they've really made it to this general corner of the fediverse, though.

You can set your instance to private and close registrations, which is what I am doing. That way you can use it only for yourself and a few friends and still be connected to the fediverse. The communities that you make on your self-hosted instance wouldn't be connected, though.

As much as I disagree with some of them, I recall the folks at the Accidental Tech Podcast spent a good while on this very discussion back when people were migrating from Twitter to Mastodon, and they had some interesting concerns and conclusions.

The answer is yes, in many places the admin hosting the content could be responsible for the content. Where I live I believe they would have to provide user data about who made the post, and if they refuse, they become the responsible party.