Funnily enough, I asked Amartya Sen and Eric Maskin about this earlier today, in addition to a similarly extensive reading list. You definitely have your quantifiers backwards.

You claim that Arrow's dictatorship criterion is "for all preferences, there exists some k such that the social preference is voter k's preference". In fact, the criterion is the stronger statement that "there exists some k such that for all preferences, the social preference is voter k's preference".

If you read the proof on Wikipedia carefully, you'll notice that it proves that:

Pivotal Voter dictates society's decision for B over C. That is, we show that no matter how the rest of society votes, if Pivotal Voter ranks B over C, then that is the societal outcome.

(Note the order of the quantifiers). They then show

There can be at most one dictator.