> Protonmail cannot do so, because your password is a part of they key to decrypt email, and they do not know it, nor they can crack it in a reasonable amount of time, so AFAIK there is nothing they can provide LE with to decrypt your email on the go.

I don't think this is true. A hash of the password could be part of the key, but Protonmail already has that hash stored on its server (otherwise they would not be able to log you in when you send the password hash). If Protonmail would like to access your emails (or be asked to do so), I'm confident that they technically can. The only benefit is when a hacker get access to the email database, but not the password hash database, then in this case the hacker cannot read the emails.