The cockpit door is set to “normal” which means that the only way back in is with a code(That way a passenger can’t get in the plane..I think this became standard after 9/11) if its set to “Lock” even with a code the pilot wouldn’t be able to get back in, that would assume that the person inside set it to lock so the pilot couldn’t get back in..no way did a medical emergency cause this this was a deliberate act
Pilot credentials can be stolen;see the link. If no one knew this copilot by sight perhaps he was killed and his credentials used by a flight trained terrorist. One of the passengers worked for an intelligence company that used satellite maps. Just thinking. About Lockerbie.