Not necessarily, Desert Dingo:
Yes, the crew could not have believed that the nav track went over Erebus. They must have believed that it went down McMurdo Sound.
But it still possible that they were briefed that the nav track went over Erebus.
Note that the briefing was over two weeks before the flight. And floating around at the briefing were photocopies of a flightplan with the McMurdo Sound nav track coordinates. I as recall, Capt. Wilson's evidence was that these were samples which were not intended to be taken away, but that one of the photocopies was not returned.
Let's assume that Captain Collins decided to retain one of the photocopies. More than two weeks later, he uses it to plot the nav track. I'm not suggesting that he forgot what was said at the briefing, but he may have disregarded it, because what's on paper tends to overide what is said, especially after two weeks.
Going back to Capt. Wilson, it was his evidence that shot the legs off the minimum altitude argument, as you have pointed out. So it's difficult to go through his evidence picking cherries, saying I'll have bit, but I don't want that other bit. He swore under oath that he told the crew that the nav track went over Erebus, and was then set upon by a tag team of lawyers - and he maintained that he told the crew the track went over Erebus.