Basil: "In order to maintain order on board there has to be an appointed Pilot in Command. At the very least a change in law would be required to appoint, say, the chief steward as aircraft commander."
In my first week at my airline (sadly no longer in business) the instructor wnet through the order of succession: If the Captain is disabled, then the First Officer is in charge. If the First Officer is disabled (dot) (dot) (dot). At one point, If the Flight Engineer is disabled, the the senior flight attended is in charge. At that point he stopped. looked at the class and said "and you're in real trouble."