I would point out that we do not know that IF a pilot was in the cabin he ever entered the emergency code. As far as I know, it is not possible for the airline to disable the emergency code option (the FCOM doesn't indicate so), so the assertion by the quoted individual from Germanwings does not make sense in saying that this option was not available. If a pilot was out of the flight deck, he and the rest of the crew could have forgotten the code (or have been told a different one to that which was programmed) which might explain why it was never entered.
I suspect the Germanwings aircraft did not have a physical "deadbolt" fitted, so that's another red herring.
Bilt mentions a GPWS "Sink Rate" warning on the CVR supposedly shortly after a pilot left the flight deck, yet this is an approach/go-around mode that you would not expect to be triggered at all in the scenarios discussed on here thus far.