>How do I view the public timeline?
Do you mean "known network" timeline? mitra-web only supports local public timeline. Other clients may be able to display known network timeline (the backend supports it).
>Also where's the config to add instances to federate with? Or does that happen automatically?
It happens automatically. Use search box to find people by address or URL, and follow them. If you want to get posts from a relay, you can create a new local user and use it to follow that relay (just like any other Fediverse account).
>Also, for a Tor-only instance attempting to talk to non-tor instances, do I need nginx set up?
No, it should work without nginx