Re: identity and encryption
Mobile email apps that don't support inward encryption wouldn't be sending the public key and would not be inviting encryption!
So thunderbird to thunderbird accounts would be encrypted. Mobile app to thunderbird and thunderbird to mobile app would not.
That isn't how email works. Email is sent to an MX server/inbox - not to a client. How the user collects their mail (POP/IMAP to a client, or webmail) is then irrelevant and entirely outside the control of the sender.
For sure, TB could embed a flag and a public key inviting the use of PGP. That's fine, and then your correspondent's TB client would send you encrypted mail by default. But you would not then be able to read those encrypted messages in your mobile app when fetching them via IMAP.
My correspondent's TB client may have received a TB PGP flag, but they're not sending emails to Thunderbird - they're sending them to my inbox, where I might collect them via TB, webmail or mobile app. The sender has no control over how I access my inbox, but of all the possible methods only TB would be able to decrypt them by default. I would have to perform additional configuration or even change mobile app to read my mail anywhere else.
This is not insurmountable - as I say, there are PGP-capable mobile apps which you could import your (and your correspondent's) keys into. It's definitely doable, but to turn it on by default is going to be a breaking change for many users.