After 40 years flying as cabin crew I had a pretty good basic knowledge of the flight deck.
However if I had been sitting in on the German Wings flight whilst the Captain was absent I would not have noticed anything unusual in the co pilot altering the flight level it would be a normal action if he had received instructions from ATC.
The only time I would have been of useful assistance would be after the initial request to enter the cockpit was declined.
I guess I could have opened the door as I am familiar with the operation of this but would that have been sufficient given the time available for the Captain to get into a position to fly the aircraft safely?
I think that two persons at all times is a good idea but don't think it is the answer to the problem alone.