Applying the rules of common sense, MSA was 14000 feet: 1000 feet above the summit of Erebus.
Applying the rules conveyed at the briefing, MSA was 16000 feet with a discretion to go lower if VMC.
The captain did not have VMC below the cloud, as he had pointed out to himself ten minutes beforehand.
He made what was, in effect, an instruments descent, backed up by what he could see out the window. But as he knew full well, the view out the window is of no assistance when flying on instruments and can be completely misleading - which is why you put the blinds down.
As for his instruments descent, there were three things wrong with it:
(1) It was against the rules, given that he wasn't VMC.
(2) The AINS was not regarded, at the time, as sufficiently precise to go below MSA.
(3) In this specific case, the AINS should never have been used, given the conflicting information about the waypoint, which the captain failed to resolve.