Second, each plane should have a dual-key activated defensive system to knock out people in the main cabin (via oxygen decreasing or some chemical agent or some other solution) which requires both pilot and the relevant authority on the ground (military? FBI?) to activate.
Absolutley not. First, remember that something designed to knock out an adult can kill a small child or baby.
Second, even with a double key system, there is still a chance of the wrong people getting their hands on the system, and I'm not talking about the bad guys either **cough ** tsa **cough**