Jacob is dead now, I think the ashes around the cabin in the past are his own ashes, put there by the losties and he needs a body.
The man in black (I think he is the electromagnetic energy under the Swan, freed by the incident and freed by Locke again by not pushing the button) needs a body and smokey needs a body too.
They use bodies of dead people now (Christian, Locke, Alex, Robert).
But with that bodies they also get the memories and experiences of the person the body belonged to.
They all need a tabula rasa, a newborn, for reincarnation.
To avoid that one of them can reincarnate the island or the statue or Jacob makes childbirth impossible.
Than Aaron is born and taken away from the island to grow up safe and Jacob is no longer in the cabin.
I could believe Jacob is a grown up Aaron, who returns to the island in the past when he is older and needs all the people he touched to make sure he will be born and taken off the island, but also knows that they have to kill him to make sure he can be born.
That makes sense

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I can't wait till it starts again.