I think if James' shoulder was injured, he would not be able to make a large snow cave by himself, which he was found in.
Yes, but that snow cave had been built on Friday night. It is possible that James was with them on Saturday for the descent and an accident happened. He could have made his way back to the snow cave they had built on Friday.
AA, I refer to roamer's post #257 in which he says the first spot found by the rescuers was likely "a protected belay position or anchor-end for a traverse", not a true cave. He even puts the word "cave" in quotes.
Then he says James survived the accident, (though badly injured) avalanche or whatever it was and "made his way to the only shelter available- The snow cave from the previous night" (leaving behind his gear).
Therefore the other cave had already been built and they spent the night before there. It wasn't very far, though no doubt tricky, and their footprints could have been followed by him back to it. But clearly he was in bad shape so no wonder he sounded as he did in the phone call.