LOST spoiler
I, too, think Jin is dead. Why would Sun be so visibly upset if it was a fake marker and he was alive back on the island. I guess she could be weeping tears of longing instead of grief, but I don't think so, at least not yet.
As for the flashforwards, they are very deceiving. We cannot assume that every time we flash forward we're going to the same moment in the future. Jack at the end of last season was obviously farther into the future than the Jack we have seen in flash forwards this year. So, we don't know the time line for any of these future moments.
The coffin was small enough for me to think that it's Ben in the coffin (and the reason no one is there to mourn him). Again, we saw Ben in a flash forward with Said a few weeks ago, but as Jack's flashforward with the coffin last year was farther in the future, it could be that someone finally caught up to Ben and killed him.
Again, all just speculation, and I should probably just stop guessing and just enjoy what they present each week without wondering how it's all going to fit together two years from now.