Ttere's no mystery at all about this accident. It was another example of controlled flight into terrain. As with most of these accidents, the main cause was pilot error. That INS was also a cause, but only a secondary cause. A failure of one navigational aid should not cause a crash.
The real mystery is how an obvious case of pilot error, once put through the New Zealand legal system, was turned into a finding that the Captain was blameless. Is anyone game enough or stupid enough to agree with that?