Shadoko - regarding slammed cockpit door with everybody outside I'd simply comment that such a situation would be an unbelievable lapse of proper procedure or issuance of an official procedure with a glaring error. Three people can navigate around each other in the cockpit quite easily. So for changing crew and assignments there would be two procedures.
The first procedure simply substitutes one person in once seat. The new person moves into the cockpit over in front of the fourth seat. The person being replaced rises and exits past the new person. The new person emerges and enters the vacated seat. At that time the replaced person exits the plane. At no time is there nobody at the controls.
The second procedure puts the captain back in his seat when the person being replaced is the person in the other seat. Captain enters the cockpit and moves to the space in front of the fourth seat. The person in the captain's seat rises and moves to the door but does not exit. The captain moves into his seat, gets settled in. The person standing moves to the space in front of the fourth seat while the captain assumes control. The third person rises and moves to the door. The replaced person moves into the seat the third person vacated. Once everybody is comfortably at the controls the third person leaves the cockpit. At no time is there nobody at the controls.
It's a simple programming job, if you think about it. What on Earth would cause all three to vacate the cockpit? (Besides, doors I've seen of late have cipher locks on them so they can be opened from the outside. Before that I believe they could only be locked from the inside by a positive action by one of the cockpit crew. If that's still the case the person planing to leave the cockpit in the above two scenarios would exit the cockpit and the last crew member to sit back at the controls would lock the door from the inside.)
(How do you put an elephant in the refrigerator? How do you put a giraffe in that refrigerator? Think about it....)