Is there a way to see a list of all of the registered users on an instance?

Not currently.

Can we restrict registered usernames (e.g. not allow slur-filled usernames)?

Yes, the slur_filter in the config.hjson will prevent any usernames with that slur in it.

But also, we recommend turning on registration applications in your server settings.

Are all Lemmy instances openly federated unless they are on the block list?

3 types of federation: open (default), allowlist, and blocklist.