Couple things I noticed: The key exchange/renewal seems somewhat centralized to some Dave guy, which I don't like too much. Also the protocol seems to require an additional, new, username for each user which seems kinda odd design for me. And lastly there is the UI problem when mixing encrypted and plain text communication in the same system.
The central authority of Dave is by design.
You are correct, mixing the should be deprecated in favor of : join unencrypted room && auth with dave && invite to secure room. But that feature is yet to be implemented.
Thank you for the feedback.