. My gut feeling is the aircraft was under some control before touchdown, and Dave would have used all his skill to achieve a safe arrival: unfortunately they didn't walk away from this one.
My feelings too. No visible damage to the MRBs does not necessarily mean they weren't turning, just that they weren't turning under power, or they never struck anything.
We haven't yet seen all 4 MRBs, it is possible that he auto'd onto the roof as per shackman's theory and there was so little kinetic energy left in the head that as soon as one blade hit something, they head effectively stopped.
We may never find out what truly happened in the last few seconds of this flight, but hopefully we may discover what caused them to be in that situation in the first place, whether it be catastrophic MRGB pressure loss, TR loss or other.