> It’s absurd that there’s no way to disable this, no option to tell Proton “if you see a multipart/signed or multipart/encrypted message, just leave it the hell alone.”

which, as I said in my response, I disagree with the first half of that. Our goal is to (automatically) encrypt messages whenever possible, and leaving multipart/signed messages alone doesn't reach that goal. So I proposed two different solutions to what OP wants, one of which is already built into Proton Bridge, and one which we could add in the future (but which would be more effort than the one OP proposes).