And I maintain that Quirrell planned out all the reasonable methods for Harry to interfere, and took steps he felt were enough to combat these methods. That they weren't enough is not something he could have known ahead of time; he was reasoning under uncertainty. We aren't reasoning under uncertainty: we are reasoning with the certain fact that he did not prepare for enough ridiculousness. He doesn't have that fact!

If you want to claim that Quirrell should not have been surprised, should have been prepared for anything Harry could do because he is that much better than Harry and that if he isn't that much better, he is holding the Idiot Ball, well... this is where the needs of the story comes in. If Harry is to be masterful at interfering and creating dramatic tension, he needs to be surprisingly good at interfering: if he can't surprise Quirrell, he can't interfere, because it will already be planned for.

I maintain that EY is doing a believable job of keeping Harry surprising, because even if a perfect rationalist had updated on all the evidence available to Quirrell so far, it could not have predicted that Harry would be able to interfere, under the restrictions Quirrell had placed on Harry.

By the way, that is where all these rationalisations for Quirrell holding the Idiot Ball are coming from. Quirrell is updating on all evidence prior to his decision and making the right decision. We're updating on evidence that comes after his decision: namely, that his decision was wrong. It is, of course, very tempting to say that Quirrell did something wrong, and that is why his decision was always wrong. But it was right when he made it! That later evidence makes him wrong does not mean he was always wrong; we are not talking facts here, but decisions.