Got this idea from someone else on here. When we see Christian after his death, he is neither Jacob or MIB. But rather a 3rd. person or entity on the island. I think he is kind of overseeing the game or war between MIB and Jacob and is trying to manipulate the outcome. Makes sense since we've seen him in the cabin. Prehaps its his cabin. Jacob wouldn't have needed the ash around it to keep out the smoke monster.