In the cover up, Jin can't be one of the two that survived the crash but died later. The date on his tombstone was the crash date.
in the investigation thread for the grave site, there is some talk of the dates, there are 3 dates Sun's birth, Jin's birth , and then the date of the crash. someone said that it could just be a memorial that was erected when the world thought everyone on the plane was was dead.
That can't be it. Otherwise, Sun's death date would be the same if they thought everyone had died and used that date.
Also, as for "hiding" Desmond ... they wouldn't have to do that. He was officially on that sailing race. The truth would work just fine for Des -- he crashed on the same island and has been there for a few years now.
But I don't think he's one of the six. I think we've seen them all -- the only thing left to find out is how we get to that point plotwise, and whether and how anyone ELSE gets off the island (since we already know Ben does).