We do know they were not expecting the track to take them directly at Erebus, they were expecting to be tracking down the flat surface of McMurdo Sound and that track was changed the night before without telling Collins and his crew
That in itself means nothing, if they could not confirm they were in the middle of McMurdo sound by means of the descent the captain requested, VMC, they could not legally confirm it with the AINS, it was not cleared for nav purposes below MSA. As no mention was made of Mt Erebus, apart from asking where it should be, or Mt Bird or even Ross Island during the descent, as recorded on the CVR, then one must assume they were not VMC and the only position line they had, the track they thought they were on, was certainly not enough. Surely the end result is sufficient proof of that?