what the other flights had done is not relevant
It damn well is relevant. All the flights were required to operate, and make their descent in either IMC or VMC, in accordance with,
Flight in the McMurdo area below flight level 160 will be restricted to an arc corresponding to a bearing of 120° Grid through 360° G to 270G from the NDB within 20 nm in order to keep well clear of Erebus.
and
The original requirement for radar monitoring of any VMC letdown was deleted by the letter of amendment detailing the conditions for VMC letdowns which were to apply following the withdrawal of the NDB letdown procedure. The revised version only called for the descent to be co-ordinated with local radar control.
this crew could deliberately ignore their written instructions re let down procedure?
Of course not. But everybody else ignored the instructions, and that is where the "Normalisation of Deviance" comes in. You obviously are unaware of the theory and what it's about. I suggest a little reading.
Briefly, Professor Diane Vaughan developed the “Normalisation of Deviance” theory in order to explain the Shuttle Challenger accident. As a result of her analysis of the Challenger accident, she was asked to testify before the Columbia Accident Investigation Board in 2003, then became part of the Board's research staff, working with the Board to analyze and write the chapters of the Report identifying the social causes of the Columbia accident. She states, "I find that in common, routine nonconformity, mistake, misconduct, and disaster are systematically produced by the interconnection between environment, organizations, cognition, and choice. These patterns amplify what is known about social structure and have implications for theory, research, and policy". One of Vaughan's theories regarding misconduct within large organizations is the normalization of deviance. "Social normalization of deviance means that people within the organization become so much accustomed to a deviant behavior that they don't consider it as deviant, despite the fact that they far exceed their own rules for the elementary safety". People grow more accustomed to the deviant behavior the more it occurs. To people outside of the organization, the activities seem deviant; however, people within the organization do not recognize the deviance because it is seen as a normal occurrence. In hindsight, people within the organization realize that their seemingly normal behavior was deviant.
It explains the inner workings of the airline at the time to a T, as it did the two Shuttle accidents.
at no time was a DME lockon achieved
It did, but only very briefly.