Very true; I've forgotten whether wizards can create false memories to cover up big time gaps. If they can, it's a much smaller problem than it looks.
But we have reason to believe that Cloak-and-hat was not expecting to have to make very many attempts, that he either is usually very good at the dictionary attack or he's that Dunning-Kruger - he got so frustrated he exploded and asked a revealing question outright. And then, the writing seems to imply, he only needed one more try to crack Hermione's code.
So, this reads to me like an expert using an effective tool who happened to run into an extremely unusual girl/problem, not a only-modestly-clever-or-perhaps-even-stupid person. (Also, thinking again on my remark that it's a very Dark technique, I'm even more confident that this is not Lupus or Sirius - neither of them seems like the kind of character to pull such a Slytherin technique.)
I'm even more confident that this is not Lupus or Sirius
It's never Lupus.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: