I think it should be followable by any actor that can receive the accept/reject activity. There's no type check on the actor, except for an inbox or sharedInbox.
Yes, it uses the relay interface either as a client (following an existing relay) or as a server (provides a fake relay to your server). The server side is kind of a fakeout; it doesn't send any data to your server by default, unlike most relays. It's just a way to get your server to send it public activities. It works with Mastodon and Pleroma style relay clients.
I'd love to talk! If you want to experiment, use https://tags.pub/user/_____relay_____ as the Pleroma relay server. Let me know how that works; I've only tested with the Mastodon style.
