Newcomb's problem isn't about decision theory, it's about magic and strange causation. Replace the magician with a human agent and one-boxing isn't nearly as beneficial anymore- even when the human's accuracy is very high.

I felt a weird sort of validation when I saw that Theists tend to 1box more than Atheists, and I think you pretty much nailed why. Theists are more likely to believe that omniscience is possible, so it isn't surprising that less theists believe they can beat Omega.

I haven't studied the literature on free will well enough to know the terms; I noticed that distribution of beliefs on free will were given in the post, and suspect that if I was up to speed on the terminology that would affect my confidence in my model of why people 1box/2box quite a lot. For now, I'm just noticing that all the arguments in favor of 2boxing that I've read seem to come down to refusal to believe that Omega can be a perfect predictor. But like I said, I'm not well studied on the literature and might not be saying anything meaningful.