Did the stuff on the island the last season actually happen?

i know the sideways universe was some sort of limbo but what about the last season on the island. did that all happen? did kate,sawyer and the group actually fly away and get off the island only to die later on and meet in limbo. so technically, the whole sideways "limbo" world happens after the island story of the last season. Also, whats with the epilogue, is that supposed to be some sort of revelation. what is with the pictures of the plane crash site.