I don't know, perhaps someone with a sophisticated 3D model of the sinking would be able to tell us. Theres no doubt that the water ingress was not even, by that I mean parts of the interior would have been under the sea surface but not flooded yet, some would have flooded from below as the water crept up and some would have flooded from above, i.e. the water may have found an easier way to the deck above and was now running down the stairs to the deck below.
In the case of the elevators, it is possible that they were flooded when the superstructure went under the water letting water into the grand staircase at the B deck level and filling up from there as described above. That far down though I am guessing they were flooded from below by water in Scotland Road.