I don’t know if it’s been tried, but if not it would be in everyone’s interest to see what Mullvad will actually cough up.
it would be the end of their business if they did, as they have a strict no retention policy. This would mean they are lying to all their customers, so it is not going to happen.
I have seen NordVPN’s response to a subpoena. Their response was that they had no records connecting an IP address at a specific date/time to any particular person.