This is interesting, though the problem it’s trying to solve is not the one I’ve experienced the most trouble with in encrypted group chats, which is handling keys on multiple devices. Both XMPP+OMEMO and Matrix seem to have quite a bit of trouble with that.
It seems like it could be too easy to end up with split groups due to distributed behavior. Say #1 removes #11 just as #12 removes #4. Does this system work without a central broker and locking?