regarding 2 pilots
The presence of both pilots will not stop one of them from incapacitating the other (fire ax) etc and still committing suicide. It is the armored door that allows pilot suicide. We don't know for a fact that the door has prevented others from taking over the aircraft but we do know in fact that it has allowed pilots to kill hundreds of innocents while taking their own lives. it almost seems that the door must be eliminated or that we have to go back to 3 person cockpits that would require the presence of 3 people at all times.