I mean that's fair, you can't trust anyone but yourself, your best bet is to self-host and hope your emails get accepted. Then again, I don't trust myself to securely set up my own email host.
Yes, whenever that Thunderbird transition actually happens! I use Thunderbird on desktop, so I'm looking forward to seeing how that goes, but it's been a long time since it was announced, and is there even a date established?
That is correct. The K-9 dev has been with the Thunderbird team for a while now and the project repo is thunderbird-android. Don't know when the rebrand is coming.