Through a parallel universe they came back. There was an older 2nd Jack that was in that coffin. That's why Jack said he was neither friend or family of the deceased. That is also why Kate wouldn't want to go to the funeral. I believe that the people who survived the flight cannot die, and that's why Jack wants to go back. Crazy I know, but it's just a theory.
I can't see why it would be jack. He brought the paper to cate from the obituary section of whatever, she said "why would I go?" Then when he was at the viewing he said it was neither friend or family, I just can't picture who it would be. At first I would Imagine Sawyer, but with the size of that coffin, I can't picture it. Plus if it were walt why wouldn't his father or family be there .But then when the guy said he had no family I thought Loocke but the coffin is still too small. I don't know, who do u guys think it was?
"That thing in the woods, It could be a monster, or just a really pissed of giraffe, I don't know" -Hurley
First of all if people who survived the flight cannot die then what about all of the crash survivors that did die, like Charlie, Boone, Shannon, Anna Lucia, Scott, Eko, etc?
Let's say only the survivors who escape the island cannot die? Then why would Jack be trying to jump off the bridge and kill himself?
Also...there did not seem to be any reaction to Jack from the funeral director when he came into the funeral home. I would expect the he would have some sort of reaction to Jack if he had arranged his body double in a coffin for a funeral.