Thank you, that clears up my confusion.

I am afraid that I am too used to intelligent fictional characters having supernatural powers of planning and foresight. I suppose it is much easier to have readers be impressed with intelligence if smart characters are simply omniscient rather than acting rationally at all points. Therefore, if you were to be writing Quirrel with maximum intelligence, he simply MUST have planned it all at the earliest possible moment. It didn't occur to me that they could just be making the best of a bad situation, since that doesn't maximize the illusion of cleverness. He's a very smart human; he's not L/Light.

I'll try not to be so hasty to make assumptions in the future and scan for any unspoken assumptions that are coloring my view when reading MoR. On further reflection, that's a good general life lesson too.