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.

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

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