Flying Head - it maybe that they were using a legacy procedure that pre-dated the S-92, something that they used to do in that area in the S-61.
The S-92 is far more capable and could have easily conducted a letdown without routing via Blackrock but maybe it was just what they were used to doing.
I start to wonder if the AIS track West of Blackrock is just an anomaly and that they never went that far away from the rock before water impact - if there really was that much damage to the aircraft after hitting the rock then it would seem to have to render it unflyable.
As for the survival aspects, it is certainly possible to release your harness and egress the aircraft yet drown on the way to the surface - I don't know if Dara's lifejacket was inflated or not.