I'm not so sure there is much of a mystery. From the report we suspect that the visibility was not much above 4km, that he might have been flying 300' lower than he thought and that they didn't fly the coast route as they said they would.
The crucial point for me is the visibility. 4km visibility, despite being above VMC minima, is bloody awful. In a Cessna there would only be about 80-90 seconds from visual of the hill/mountain to impact. Clearly it is conceivable that some workload issues and the attendant distractions would reduce this time to much less than not very much, conceivably not enough to avoid. Especially considering the potential altimeter error, it is highly likely that the minute required to climb the 500' needed to clear the hill would be insufficient. The runner does say that the engine pitch seemed to increase just before the accident.