> With Signal, both endpoints are Signal-controlled (yes, the apps are open source, but at least on the App Store and Google Play you can't verify that the open source code is what's shipped).

While I agree with the concern here, is there an alternative mass market model in which software can be distributed in a way that ensures no one but the user controls the device? I mean mass market, so the barrier is that my tech illiterate grandma has to be able to install this with the help of my tech illiterate parents.