I think those "fast forwards" are not really fast forwards from the show's current reality. Remember in the current reality, Jack's dad is dead yet in the "fast forward" he refers to him as being alive. Therefore it could be anyone in that coffin because we havn't been watching those scenes from that alternate reality. ....or have we?
Also, why is Walt two years older when Lock sees him at the end of the show? (beyond the fact that the actor grew older) If the visions of dead people everyone sees on the island is just the smoke monster morphing into them, why did it not choose the age of Walt when he left the island? If visions of people everyone sees are only of dead people, does that mean Walt is dead too?