Following this thread with interest as someone who works in the skies. I have nothing of value to add, just to clarify sop procedures with regards to flight deck entry (at least with my airline, couldn't and wouldn't want to speak for MA).
Normally one of the crew (quite frequently the one or two working in a position near the fd or the cabin manager) will ring the flight crew roughly every twenty mins/ half hr depending on service to offer a drink, check all ok. In our case we have to say a pre agreed password (normally agreed at start of flight pre boarding) so no prior knowledge of this. The flight deck are then expecting the crew member who will only approach the fd door if no pax are in the vicinity,the door will be unlocked by one of the flight deck after checking the camera to make sure all is 'as it should be'. If the crew member entered the emergency code to enter the fdeck, say assuming they thought the pilots were somehow incapacitated the pilots have a certain period of time to deny entry, ie lockdown because they think the crew member has a gun to their head. Lockdown would remain in place until the ac landed.
In a lot of ways the locked flight deck door has, as mentioned before I think, caused major concerns and I know lots of flight crew who don't like it because it's too easy for a kamikaze/suicidal/unconscious pilot to have the time/means to do something horrific.
Crew are always taught the you 'protect the flight deck at all costs.'
I just can't bear the thought of what the crew,passengers and families are going through.