the ONLY reason (repeat ONLY) for the presence of a 'replacement' person during any brief absence by one of the two pilots would be to over-ride the authority of the pilot-in-command at the time regarding door operation.
Is that what all these folk intend? Making a member of cabin crew 'Captain' seems a strange, illogical solution.
Not least because it requires that CC member to make a quick, accurate, on-the-fly diagnosis of the difference between a calm co-pilot intent on crashing the aircraft while the captain hammers on the door overcome by desperation (GermanWings, apparently) and a calm co-pilot intent on saving the aircraft while the captain hammers on the door overcome by delusion (JetBlue, as I understand).
How? Why would someone with those skills be CC, not printing money in Harley Street?