If they offer IMAP, they would also need to decrypt your data on their side, which I'm happy they go to lenghts to avoid.

This is a case of Proton sacrificing convenience for privacy. If the former is more important for you than the latter, there are better alternatives out there.

I don't know, and it's why I moved off them. Additionally, the Bridge is only available for Windows/Linux/Mac, so if you want to use your own email client on mobile, I guess your only option is to VPN into your home network and expose a bridge instance there or something. No hard feelings, but I can't really recommend them as an email host if what you're looking for is... an email host.