You can't make up just one scenario and its result and say that you have a voting rule; a rule must give results for all possible scenarios.

I think I see how the grandparent was confusing. I was assuming that the voting rule was something like plurality voting, with enough sophistication to make it a well-defined rule.

What I meant to do was define two dictatorship criteria which differ from Arrow's, which apply to individuals under voting rules, rather than applying to rules. Plurality voting (with a bit more sophistication) is a voting rule. Bob choosing for everyone is a voting rule. But the rule where Bob chooses for everyone has an a priori dictator- Bob. (He's also an a posteriori dictator, which isn't surprising.)

Plurality voting as a voting rule does not empower an a priori dictator as I defined that in the grandparent. But it is possible to find a situation under plurality voting where an a posteriori dictator exists; that is, we cannot say that plurality voting is free from a posteriori dictators. That is what the nondictatorship criterion (which is applied to voting rules!) means- for a rule to satisfy nondictatorship, it must be impossible to construct a situation where that voting rule empowers an a posteriori dictator.

Because unanimity and IIA imply not nondictatorship, for any election which satisfies unanimity and IIA, you can carefully select a ballot and report just that ballot as the group preferences. But that's because it's impossible for the group to prefer A>B>C with no individual member preferring A>B>C, and so there is guaranteed to be an individual who mirrors the group, not an individual who determines the group. Since individuals determining group preferences is what is politically dangerous, I'm not worried about the 'nondictatorship' criterion, because I'm not worried about mirroring.

I'm not going to rewrite Arrow's whole paper here but that's really what he proved.

I've read it; I've read Yu's proof; I've read Barbera's proof, I've read Geanakoplos's proof, I've read Hansen's proof. (Hansen's proof does follow a different strategy from the one I discussed, but I came across it after writing the grandparent.) I'm moderately confident I know what the theorem means. I'm almost certain that our disagreement stems from different uses of the phrase "a priori dictator," and so hope that disagreement will disappear quickly.