This may seem simplistic, but have you put the key in the outside door locks and cycled them? Wiggle them around before removing the key to make sure the lock is centered in the unlocked position.
I was chasing a VATS issue a while back and discovered the passenger side door lock was not making good contact.
A long shot, but not hard to try.
I assume you have checked for any codes present? Cleared if so? Disconnected and reconnected the battery?
Good luck.
Marty, Looks like you posted as I was typing. Appears we are both going the same direction.
Happy Thanksgiving,