Honest question, because that's the obvious difference (if you believe ProtonMail claims).
There is your answer.
What I have no idea about is Google. Do they even need to do anything targeting you in order to decrypt the data or not? Obviously they can modify your software with a remote update such that they can capture your decryption password, but that's a lot more work than querying a database and using a master key that Google has on hand for this sort of thing.
While (as you correctly pointed out) the fact that you lose access to the data if you lose your passphrase doesn't prove that the data is completely inaccessible without it, the opposite (being able to access the data even if you forget/lose all your secrets/keys) conclusively proves that such access is possible.
You can't. But this really doesn't answer if there was a special way for law enforcement, which I don't believe there is.
Obviously they can modify their software to capture your decryption password, my question is more out of curiosity than it being a serious advantage against a state actor targeting you specifically.
I am fairly sure that protonmail cannot do this without modifying the software to target me by capturing the decryption password.
Of course they can always capture plaintext messages as they come in, we can only assume they don't keep records of that. If that's true, and we only have their word for that, it would make any requests forward focused, like a wire tap rather than a search of old bank records. They can't necessarily access old emails without explicit effort.
It's not some insurmountable barrier, and I don't mean to suggest it is. It's trivial to think of at least three ways to work around it, assuming you are still logging in. But it is a difference in design.
