Also, if the workload is too great for a single pilot, is it possible for a single private pilot to land the aircraft with the assistance of a flight attendant or second passenger?
It seems like someone could be told, by radio, how to reduce speed, lower flaps and landing gear and even operation of the thrust reversers if the individual had flying experience.
The alternative to making no attempt, of course, would be death for all aboard.
"Assuming the deaths or incapacitation of both pilots, is it possible for anyone to even enter the flight deck if the door is locked?"
I doubt it. I wouldn't try it anyway.