Good discussion (mostly) so far.
Teropa, much of the situation you pose so far I think would work.
The two most likely snags that I can see are:-
- In a case of double pilot incapacitation, the cabin crew would be very unlikely to ask if there was another pilot on board, mainly to avoid seriously upsetting all the pax. If they knew there was one on board (which is quite probable) they would have a quiet word with him. They would never ask if there was an experienced FltSimmer on board. Therefore you'd be out of the loop.
- Assuming you get into the flight deck, in most parts of the world it would be almost impossible for an ATC unit to locate and bring to the mike a suitably type-rated pilot. Assuming that they can, time is going to be very short. You won't have much fuel to hang around while they bring a guy in from his gardening. If he's not from the same company as your aircraft, he quite possibly won't know the equipment and layout of your flight deck. (e.g. Most British 757's have paddle switches to engage the A/P - American versions have pushbuttons)
That having been said, and given all the premises in your scenario happen, I don't see much reason, given a fully-functional aircraft (no "Autoland system U/s" in the tech log etc.), why you shouldn't be able to push the right buttons to get it down.
PS Yes, I'm 757/767 type rated.