Just to add/reiterate a few points:
ICAO RT doesn't need to apply in this instance. The US FAR-AIM doesn't require MAYDAY calls to declare an emergency. (ICAO would be much better, and maybe this very flight will change things.)
The Pilot used a shortened callsign, but the controller used the exact same shortened call sign when he told the flight to expedite clearing. If the pilot had used the callsign Acey 5912, the controller wouldn't have thought "Acey United 12???"
It seems to me the controller's bias against believing the odd sounding call (O2 mask) came from any real aircraft bears a large part of the blame. Perhaps this even lead the controller to think it couldn't have been 5912 that made the call since he sounded normal (boom mic) when checking in.