As I said at the outset of this thread, the chances of both pilots being out of service is minimal.
IF that were to happen, would the airline call upon the Purser or other willing member of staff to push the buttons as directed - or would they gladly accept the offer of a PAX?
Cabin crew are a known entity and under the direction of their employer. PAX volunteers are not. It is better for the airline to have a member of their company crash the machine than to allow an untrained person to crash it.
I realise that the question is not if such a circumstance could arise but could the PC person handle it but let us be clear - the situation will not arise.
I have to say, from the point of view of a pax that has been flying regularly for 36 years - I would not want you anywhere near the flight deck.