You need to understand MSA......The Nav section didn't plot a course below area MSA, there was nothing unsafe about the route they gave him on the day.
The route they gave him was over the top of Erebus, an active volcano prone to persistent Strombolian eruptions. Chippendale in his recommendations said,
No commercial passenger carrying flight be planned to fly over or close to an active volcano
Somebody finally realised the danger presented by the ash cloud, and bombs ejected from the crater. ie It wasn't smart nor effing safe. There was a reason the RNC track went down the sound, keep away from Erebus, and that's the route Chippendale recommended.
Eruptions occur on average six times per day, and have been known to throw ten metre diameter bombs up to one kilometer above the crater rim. Mountain 12,450 feet plus one kilometer = 15,731 feet. Now, what did you say the MSA was?
Again, it's a demonstration of the lackadaisical approach taken to the entire operation, people not knowing what they didn't know, and not enquiring enough to find out. Bit of a "We invented aviation" mind set?