OK, I am beginning to see a problem.
AirNZ appealed Mahon's finding that there had been a cover-up. They appealed to the Court of Appeal. The Court of Appeal upheld their appeal. In turn, Mahon appealed to the Privy Council, whereupon his appeal was dismissed. The appeals were only ever in relation to Mahon's claim that there had been a cover-up.
It is very common for people to assume that (and then praise) the law courts for "overturning" Mahon's conclusions as to "cause." What adds a certain irony is that these are the same people that tend to criticise Mahon for being a non-aviation judicial officer making some big calls in areas he knows little about. That may be so, but that is the nature of a Royal Commission. The Commissioner(s) need only take advice from specialists, which of course Mahon did.
The Judicial Committee went out of its way in praising Mahon's "painstaking" work and even stressed that the question as to cause was, in fact, not put before them. You can read the entire judgments of both the Court of Appeal at
www.gutenberg.org (search for erebus) and I think you can access the Privy Council judgment online also. If you can't find the PC judgment, I will email it to you.
I have to say that your arguments have been impeccable thus far, but I do have to question your claim that the accident crew complied with none of the four points in the briefing paper. On the first point, there was no evidence to suggest that the visibility was less than 20KM, on the second point there was no evidence of any snow showers in the area, on the third point the aircraft was not equipped to receive azimuth signals from the TACAN, the only way they could have complied with this instruction would have been to use the INS (a task it was not set up to do, nor were the crew instructed in any way on ANY methods using the INS, the only possible method left was VMC, and this was what the crew were doing), and to the fourth point, the meaning of the requirement was only that descent be coordinated with the local radar controller. This was, in fact, taking place with the radar controller at "Ice Tower" through the "Mac Centre" controller on HF. Radar identification was not a requirement of the briefing condition, although there is no doubt that it should have been.
This leads me to an interesting question. Just what is a "VMC descent?" In New Zealand, the closest thing is a visual descent below MSA. The only way you can do that is by conducting a visual approach, or by radar. I agree that neither of these things applied to the accident flight, therefore I do not know what was going on there.
There should have been a clear cut prohibition on descents below 6000ft if that had been the intent of the briefing. That people who were there can argue about what the briefing said (or meant) attests to its complete failure to provide anything of the sort.
Briefings are supposed to assume that pilots are stupid. A safe system also assumes that pilots are stupid, and goes out of its way to help them discharge their responsibilities. In this case the system badly let down the captain and crew of this flight. You can argue all day about who carried the responsibility yada yada yada, but ultimately 257 people died because the captain was unable to outwit a stupid and flawed system that covertly obstructed him at every turn.