I think this explanation still fits in with the scenario of them having trouble descending and his seeing his friends lost. If he was hypothermic by 12/10, his ramblings would make some sense. But... if he died shortly after that call, how did the Tuesday cell phone turn on happen?
You would have to get a better definition of what "he died not too long after" means. What is "not too long"?
Earlier on this thread I posted that I think James died close to the time the cell phone was turned on, then went off. I think he turned it on as a last act before losing conciousness forever, as a way of reaching out to his family.
I would define that as within the framework of "not too long after". But someone else, including the medical examiner, might be thinking it was within hours or minutes of his conversation with his brother. That might be, but would leave the question of how did the cell phone get turned on sometime Tuesday.
Maybe when the water got into it? A short?