The difficult that most conspiracy theorists face is that their one conspiracy is never enough, because contradictory facts exist. So to continue with the theory, the conspiracy theorist has to postulate a further conspiracy, and so on.
Before labelling the AirNZ witnesses as perjurers, why didn't Mahon try and reconcile their evidence with that given by the line pilots? If you have a close look at Capt. Wilson's evidence and the evidence of the three surviving pilots, you can, in fact, reconcile the evidence, by finding that Capt. Wilson said that the nav track went direct to the NDB at McMurdo Station but also finding he didn't say that the track went over Erebus. If looked at like that, no-one told an outright blatant lie on the subject (although Capt. Wilson might have gilded the lily a bit by inserting various references to "over Erebus", and the three surviving pilots would definitely have sailed extremely close to the wind.)