With lethal defense strickly limited to cockpit defense AND behind a hardened door that gives the flight deck crew to assess the situation, there is really no need for "instant" decisions. If someone is trying to break through the hardened door, it's going to take awhile. Authorized entry/exit is easily handled procedurally, and/or with physical/sight barriers to prevent opportunistic attack.
One more time:
1. Lethal cockpit defense is the last ditch defense when all else has failed.
2. Loss of command of the aircraft = mass death and destruction
3. Lethal defense is limited to defending the cockpit and retaining command of the aircraft. Shooting a man size target, with prior warning, at 4 feet trying to breach a door doesn't require exceptional marksmanship.
4. Hardened cockpit doors give the aircrew time to present a defense prior to breach.