We'll see. not at first, but if you can have a nostr client outside of Breez, you can also have it Breez with no or a neglectable performance degradation. 

do you imagine it as a full-fledged nostr client with global feed? Or maybe just a few group chat rooms? 
Not sure if you've noticed but nostr clients are super busy, opening connections to bunch of relays and maintaining local db etc so it's not lightweight and could be a slow experience.

zman

#3 is bigger but nothing messier about it imo. We've done it successfully with our POS & podcast player. The app is built for extensibility. 

nostr

Nostr in Breez: call for Nostr devs

1- nip7 support should be pretty straight forward to add and then apps will get window.nostr support.
2- basically replicate https://github.com/fiatjaf/nocomment functionality.
3- trickier and possibly messier. I vote it's not a good idea unless there's some benefit of having it inside breez. it would be like adding a shoutbox.