F-Droid on Nostr: Yes, either the developer themselves apply for inclusion (the best option, because ...

Yes, either the developer themselves apply for inclusion (the best option, because then they are totally behind it), or any other person can apply for any app (then the original dev has at least to be notified about it).

Regarding ProtonMail, that's because both the "old" and the "new" Proton Mail apps depend on Google proprietary libs.

Unfortunately, the devs Proton (npub19kd…t56d) didn't have time yet to build a completely open source variant of Mail.