They didn't die in the 815 crash, it all really happened, and they died when we saw them die(Shannon by Ana Lucia and Charlie drowning in the Looking Glass for example).
The line about some dying before Jack died and some after gave it away. And Christian actually said it happened too.
Hurley would have died after Jack, some point during his island babysitting with Ben.
Then they all turn up in this self created alternate reality so they can meet and move on. As they said the alternate reality existed somewhere where there was no time so they all ended up there once they died.
I assume Kate, Sawyer etc..had a life off island then once they died turned up in the alt to move on with the rest of them.
Should have guessed when we saw David as in pre-815 crash Jacks wife had a miscarriage so this is the son he would have had.
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