[deleted] I've just tested the application
Oh, you meant the Android app, not the apps provided by Proton, which does support FIDO keys.
I guess they would have to open a browser webview to communicate with the hardware key. The benefit of FIDO over OTP as 2FA, is primarily phishing resistance, which wouldn't matter when using a native app anyway.
[deleted] Google implements fido keys much better than proton
Google, having deep integration with Android phones, would of course want to make security keys work within the non-web apps. Does this require Google Play Services? If so, that's a deal breaker for many.
[deleted] If someone sets all their passwords in protonpass, and sets the 2fa code in protonpass, then the applications become unusable if you uninstall and reinstall them
I don't understand what you are trying to do.
[deleted] If you set APP in your Google account, even Google can't open your account, unless there's a flaw in the fido protocol, which I highly doubt.
Are you saying you've tried Google Account recovery? Google does have the ability to reset/turn off 2FA.
If you want to test this. See if you can reset your Google Account without 2FA using a recovery email. If it is possible... then Google can absolutely access your account.