Once again posters are just NOT reading the foregoing 3244 posts.
Pace - there is NO need for the 'replacement' crew member to be trained to open the door in normal circumstances:
1) A fully equipped 'normal' door can be opened easily and safely by the remaining flight crew member from their seat
2) Should this flight crew be physically unable to do so, there is an emergency ingress system to allow access from the cabin
The only time the 'extra' crew would need to open the door is if THEY judge it is necessary to over-ride the authority of the Captain of the aircraft (the remaining flight crew) where they do not wish it to be opened. This is the major issue most of us have with the system. It opens a huge can of worms.
I don't think it has still been clearly established exactly what happened on the JetBlue flight where the nominated Captain was 'locked out' by the F/O, whether there was an extra crew member in the cockpit and how the situation was handled by them.