Prospector:
"Sucker hole" is exactly the right expression. It was a nest of vipers down there, which is why it's difficult to identify an error. All there is is the Beaufort Island issue, and although Mahon's Dunlop Island theory and MacFarland's 'gap in the sea ice' theory are both nonsense, you still have the mindset and the time pressure.
Which is why I have tried to look at the situation before they descended down the hole. It looked quite straightforward: Crew failed to verify position before going below height of known hazard. But it looks as if they intended to verify their position before going down, and then changed the plan and descended through the hole. The only explanation for that that I can come up with is that second memo.
So it's starting to become a bit of a viper's nest above the cloud, as well as below.