I think this topic is starting to drift off course...Terrorism might be possible, but there has to be other logical explanations. The FBI stated that they are only involving themselves as there were American citizens on the aircraft, and the NTSB are interested as it's an American-assembled aircraft .
I'm sure the saying "Aviate, navigate, communicate" applies to commercial flying as well - if the crew found themselves in a time-critical situation with a high workload, perhaps they were too focused on controlling the aircraft to perform a proper Mayday call. That said, few scenarios would be that time-critical at 35 000 ft, so it is all certainly a mystery.
That's my 2c at least, I know my priority would be to take control of the aircraft.