Lots of good ideas, though having either a Cockpit door that either couldnt be opened from either side without it being on the ground (where Ground Crew could access the suggested exterior lock, OR a solid Bulkhead making the Cockpit totally separate from the passenger compartment, wouldnt be that assured (though of course FAR safer than now). Terrorists would simply insist that the aircraft be landed at a remote airstrip where they could access the Cockpit, & force &/or replace the Cabin Crew.

My suggestion might be that the Flight Plan that is presumably already input into the 'flight computer' as an aid to navigation, could be programmed to PREVENT major course deviations & landing anywhere except at the actual destination (OR point of departure, as a safety measure in case of mechanical, etc, problems with the aircraft).

Legitimate deviations from the Flight Plan (made necessary by bad weather at the Destination or whatever) could be made by coded computer signals from Air Traffic Control.

Control Towers are more easily defended than Aircraft are.