1. Not as much benefit for 1-on-1 except that you can guarantee connectivity without a STUN server, can get a bit more predictable network paths from client to server than client to client, and can abstract choosing the best simulcasted stream quality. It won't double the connection count, but it can make you upload unnecessary streams in the case of simulcast, because the client won't be the one determining which stream quality is best, it'll just send multiple.

2. Yup, SFU acts like another client. Some libs do one connection for up and one for down or other ways to handle stream multiplexing, but overall it's the same WebRTC peer connections.