Couldn't you use HMAC with shared secret key to authenticate messages while keeping plausible deniability? Since the key is only supposed to be known to the 2 parties, the recipient can deduce that a message was actually sent by the sender if he did not create it himself. I think that's what OTR was using.
F-Droid is like package maintainers for Linux distributions. You have to trust them if you want to use their pre-built binary. However, some app builds are reproducible. Check the doc for more info.
DroidFS can open existing volumes. Just click on the "+" button in the bottom right corner, then on the folder icon, and then select the volume directory.
Ah yeah that sucks, but you can access it from another lemmy instance over federation, like so: https://lemmy.ml/c/monero@monero.town