What happens if someone with old Riot client creates a room and someone with e.g. RiotX joins it, will it force E2EE on? Or will it fall back to non-E2EE messaging?
IMO it should be the case that it's always E2EE, no other options. Until that's the case I think Matrix ecosystem isn't keeping up with centralized solutions like Signal.
My only big issue is that the iOS client doesn’t support multiple simultaneous identities.
If there are other applications that can run over the protocol, I'm interested in learning about them.
You could connect to a pub — an automatically-friendly bot account; see a list at https://github.com/ssbc/ssb-server/wiki/Pub-Servers — scuttle.space seems to be active right now.
If you're happy posting your SSB ID publicly, I'll follow you, and that may help. Or you can use the #new-people tag if you want to introduce/announce yourself :)