I'm going to stray even further into blind conjecture about edge cases here.
Supposing interest in a ProtonMail user and faced with the potential molasses-slog of going through the Swiss legal system, it's possible -- maybe even likely -- that the U.S. government would opt instead for illegal methods of compromising ProtonMail's servers.
Again, the most they could theoretically gain from this would be ProtonMail's private keys and thus access to the metadata listed above, but not access to the plaintext of any emails or any user's decryption password (which is never sent over the wire).