Your tone and your assumption that everyone else is an idiot is irritating.
The key part of your first sentence is "via Proton". Support for client side gpg is easy and they're not doing it either out of some strategic play or purely out of stubbornness. Working on standarts is great! I've had a "Visionary" subscription to Proton for years, since before the VPN and all the extra stuff. I like the company, overall. But, as mentioned in my first comment, this is the singular most annoying part of their service to me.
Internally, yes. So, they only allow it if it's under their control. This wouldn't be a customer servie nightmare because only people who know how to use it would use it. Plus, their version of PGP doesn't encrypt the subject.
It's a simple ask, not bending over backwards. I bet they haven't touched the email encryption part of code in years, so it doesn't add any maintenance burden either. I've looked at what they do - the only thing they'd need to change is their handling of email headers!
Why is that a fault in logic? The features are orthogonal. One doesn't restrict the other. All other, normal, email providers allow client side gpg use.
Afaik, there's no way to get your password from the JWT and access to account, right?