"I don't subscribe to the theory of someone else hijacking the plan because by know the backgrounds of all those on board would have been examined and anyone without a pilot's licence learning to operate a 737's navigational and comms system would be suspicious...."
Agreed.
And besides to hijack a plane from outside the flight deck especially a 777 you would need a while team of people with equipment. Not just to get into the flight deck but to defend and ward off cabin crew and passengers in a counter attack - which after 9/11 would guaranteed.
Also they would have no way of ensuring they could gain control at a time where fuel resources would allow diversion to their chosen destination.
And any attack on the flight deck from out side would have resulted in mayday calls.
The cards are all starting to fall the one way.