I had the very same problem,tried everything that you did plus heat also trying to get to the screws that held the bracket to the vent edge.
I even called MaxxAir,they had no answer except to cut the wings off the cover. It seems that the wing nut was galled to the stud and the stud is press fit into the landing.
My final solution was to take my small air powered cut off tool and grinding it down so I could pop it loose. To Maxxairs credit after 2 calls trying to find a remedy they sent me replacements free. I then installed them after putting on a good coat of anti-seize.
This may one reason that the new hinged ones have a pull pin latch like the new
Fan Mate.I did change mine out to the new model so that would not happen again.
