MR - to be fair, the more we hear about this sad accident, the less it looks like hyd failure/servo transparency but it was brought up at the beginning of the thread by a very experienced test pilot and investigator - it should still not be totally discounted, especially as a contributory factor.
A hyd failure might be easy to diagnose and manage on a PC/OPC when you know it is coming and during daylight - it would be a whole different ball-game at night in poor weather when you were already working hard to keep orientated.