Ah totally glossed over your second question about Nostr, lol. I suppose it would just need to be really normie friendly... like just them scanning a QR code you provide to start. That would then invite them to a "managed" or sub account (can a nsec generate another npub address?). Ideally they wouldn't have to configure anything to start other than a name and optional profile pic. They should inherit your relay setup, etc. The tricky bit is I don't know enough about the Nostr protocol to know if it's possible to export someones notes to a new identify? I'm guessing probably not, but at some point they'd want/need to be able to manage their own nsec.

What are you thinking for the actual function of it? I don't see the point other than following each other and maybe having their follows as suggested follows.

I can't think any real use for F&F onboarding in clients.