I get your need to pigeonhole this as Groupthink. You've invested time and reputation on it, so no wonder. Let's look at something obviousPete, take a look at the first symptom of groupthink (below). If he was deluded... WHY was he deluded?
He was an ENT. Line up nine deco instructors next to an ENT and ask another ten divers who they are going to listen to when it comes to physiology. It would be him by a landslide and why not? He's got a real bonafide degree with years of study behind him. In other words, he was the defacto expert.
As an ENT, he was the one to always have the answers. He was charismatic on top of that and was able to convince a lot of others that he was on the right track. Sure, he had a posse, but they were the bus drivers, the sherpas and the worker bees. Rather than being a product of Group Think, this was the product of a maverick: a rogue diver. He even believed his own hype and gave his life trying to prove that he was right. Delusion and denial for sure, but no matter how you try to assign culpability to others, it just isn't there. I just don't see your 'Group Think' here.