I do not think they replaced it
Most probably, it has just not made its way into alpha yet. At the very beginning of alpha Swen mentioned in one of the numerous videos, I think, that in the alpha build you begin right on the beach without any introduction. There should be a proper introduction in the final game.
No, it's clear from context that Original Sin's "survivors of an attack" hook was replaced, given that you've got official orders and such. I don't mind the murder investigation idea, looking for clues, that's a bit more purposeful and interesting than the mostly generic wandering from Div 1 and 2.
(Divinity 2 did have you do some things, but only until the ritual in the church basement. After that you're left to wander about until you eventually leave the starting area.)
