The only damning evidence of Christina being dead is that Jack saw his body in the morgue. Tough to get around but I can with some hollywood magic. GIve him some drugs to lower his breathing and bloodpressure. Jack only looked at him for about 10 seconds tops and he broke down. So he could have been fooled.
I also believe the Christian on the island are visisons from dehydrated Jack. Not christian running through the jungle.
Reasons he is alive.
1) Airline worker was argueing that Jack did not have the documentation to get the body on the plane. But the casket was on the plane. How did it get on the plane? Well an empty caket does not need documentation. No body was in the casket so it was put on the plane.
2) Suspecious hotel room, with the cigarettes and medicine and the wallet. All clues to the truth. Very cryptic clue in Jacks question of who would leave their wallet behind.
3) The producers saying that if you die on the island, you stay dead, Christian did not die on the island, he is alive
4) would make for a great story line.
5) Christian is so involved in getting people on that plane and on the island. Jack, Claire, anna ... he is very involved in this drama unfolding.