@alex @3baabe4d008648b5438bfe3180e3d443cbc36478fbabb65007099a96a1c56922
I've been researching and planning to write an XMPP-Nostr gateway and ran into the same issues and had similar ideas to you.
The signing requirement makes Nostr incompatible with other protocols if you want the user to control their keys and your gateway to not know them.