Just use Proton
You're also wrong in the aspect that it would undermine something, you can absolutely export keys from Protonmail, you just can't use your own keys properly. You can't remove all the keys they have generated, you can't use your own client with your own keys, the bridge literally mucks it up. The defaults can be what they are, it's not mutually exclusive in any way.
In the end this restriction undermines the security and privacy for everyone that want to use secure hardware storage. Which is absolutely insane for a service that boasts about these things.
They shamefully don't care about security and privacy because you can't get anti privacy capabilities working to your satisfaction.
You apparently could have lead with a lot of valid complaints but your 'shame' isn't really consistent with what you actually want.
Both the IMAP bridge and web interface should provide original unmodified emails upon request.