I would bet my left boob, that Jack does not die in the fire, at least not directly. The stuff on Rebecca's seat as she drives past the burned out house are not things a man would have on him in the middle of the night as he grapples with a fire. He would have his wedding ring on, but he wouldn't being wearing a watch or have keys and a notebook. My guess is that Jack gets out of the fire just fine, and he goes on a bender and something happens, or he goes back to the house to look for the dog and a beam falls on him or something. Yes, I know Kate has the dog in the Miguel house scene, but the dog could have found her as Jack was still looking for it.