he had to descend through cloud that was already below MSA
There is no evidence that he descended through cloud, in fact there is no evidence that he didn't maintain VMC at all times. I don't know the regs applying at the time in NZ but over here descent below the LSALT if in VMC is permitted and is the procedure used when flying to an airfield without any approach aids.
I want to know why not seeing it (a very nearby, tall mountain) didn’t ring alarm bells
We'll await the answer from someone with a crystal ball.
Did the airline have an SOP requiring the plotting of waypoints derived from the AINS? My assumption is maybe no, as it wasn't until the penultimate flight that questions were raised by it's Captain, which set in train the waypoint change for the final flight.
For those who play the blame game, who would you blame here? We lost an F-111 which crashed into an island on a low level night flight, many parallels to Erebus in my eyes.
https://static1.squarespace.com/stat...ward+Short.pdf
RIP
Flt Lt Anthony "Shorty" Short (pilot) and Sqn Ldr Stephen "Nige" Hobbs (navigator)