Errrr how is aviation security going to help in the 3 instances quoted?
1.cargo hold door is left open and aircraft takes off only to make tech return about ten minutes later.
2.smoke comes out of cockpit during flight due to technical problems,and passengers claim to have seen flames comming from one of the engines.
3.physical altercation with paxs trying to leave aircraft by the airline crew.
In situation 1 I suggest a better walk round check by the ground engineers seeing the flight off would be more useful than a non qualified security guard. Even better would be for the loading staff to do their job properly!
In situation 2 it was during the flight...............so where is the security guard sitting? On the wing? well you'd need two then, one for each wing
In situation 3 there is a possibility that having a security guard at the aircaft might have been useful. It isn't clear at what stage in the proceedings the pax tried to leave the aircraft.