The End (2) - S6-E18
Continuity mistake: As Locke stabs Jack's neck in their final battle, the wound is visible, above the neck line of his shirt. Once the rain stops and Jack speaks of his plan and kisses Kate, the wound is missing for the entire scene. The wound reappears on his neck during Jack's scene with Hurley and Ben at the cave opening. During the final sequence as Jack lies down and closes his eyes, the wound is visible in most shots, but in the final two wide shots, the dry blood make-up appears to be coming off and once again there is no neck wound.
Chosen answer: None of the main characters died in the plane crash. Many died on the island after the crash (Jack, Charlie, Sun, Jin, Daniel, Juliet, others), some lived on the island for an apparently long time after the crash (Hurley, Ben, Bernard, Rose) but some lived lives off the island after the crash (Sawyer, Kate, Claire, Miles, Aplert, and Lapidus). Remember that Christian tells Jack that "time has no meaning here," (in Purgatory). When everyone meets at the church, they are at the end of their lives however long that may have been and will now "move on" together. They look like they did on the island because that is the way they best remember each other.
That has to be the clearest explanation of the ending I've ever read.