You still have not got it.
though even 6,000 VMC was a fools paradise
To even descend that far they had to have been identified by radar, had VHF contact, and have a DME lockon, they had none of these things. And who was it that made the decision to descend?
They also had company memorandum that covered requirements for descent OAA:14/13/28 dated 8th November 1979.
Delete all references in briefing dated 23/10/79. Note that the only let down procedure available is VMC below FL160 (16,000ft) to 6,000ft as follows:
1. Vis 20km plus
2. No snow shower in area.
3. Avoid Mt Erebus area by operating in an arc from 120 grid through 360 grid to 270 grid from McMurdo field, within 20 nm of TACAN CH29.
4.Descent to be coordinated with local radar control as they may have other traffic in the area.
Given all those facts I would have thought anybody or perhaps I should say any reasonable person, would accept the Company and CAA had done their bit to ensure a safe operation.