I'm not interested in the altitude issue and the various alleged rules. Let's just say that the captain was subject to the general rules of good airmanship.
The track issue is different. The first correct statement by megan is that the nav section did not know that the final waypoint was 20 miles west of McMurdo Station. Absolutely correct. They though the waypoint was at the NDB at McMurdo Station. Captain Simpson noted the 20 mile difference and told Captain Johnson that the waypoint might be better positioned at the TACAN, which was a couple of miles away from the NDB. The nav section obliged, thinking they were only moving the waypoint from the NDB to the TACAN.
The track diagram you refer to was not in the briefing papers handed out at Captain Collins briefing. It somehow found its way into the briefing papers given to Captain McWilliams a couple of years before, and showed the military route to the Byrd reporting point followed by a left turn towards the runway at Williams Field. But the military route was designed for a landing, The Air NZ route was not. It's a complete red herring of no relevance.