It seems that the popular opinion around here is that Mr. Hat & Cloak is someone, anyone, other than Quirrellmort. I think this is a case of the same kind of thinking that led people to wonder whether Quirrell was Voldemort a lot longer than Eliezer intended.

I think Eliezer probably meant us to know that Quirrell was H&C the very first time he appeared. Quirrell follows after Zabini when he leaves Harry; Zabini says that Quirrell reacted exactly as H&C told him he would. He knew how Quirrell would react because he is Quirrell, and he told Zabini to do what he did specifically so Harry, who Quirrell knew would be around after the ceremony, could hear it and have another reason to distrust Dumbledore.

Eliezer has already dealt with this once. Everyone suppressed their own knowledge of canon and faculties of logic even in the face of nigh-incontrovertible evidence that Quirrell was Voldemort. He expressed his confusion at this in the author's notes, and I believe he vowed to make his blatant hints more blatant in the future.

I think Quirrell being H&C is even more blatant than Quirrellmort was, and here we are doing the exact same thing. We do it because we love the story and want to preserve as many surprises as we can for as long as possible. We want to wait, wait, wait for a bigger payoff later.

I think Eliezer is likely to be silently rolling his eyes at us on this, thinking "There they go again!" and chuckling quietly to himself. But I know that if he is, he's darn well going to let us figure it out for ourselves this time, rather than once again holding our hands as we step gingerly to conclusions to which he meant us to leap.