Dumbledore is now a much more interesting character-- what mental resources does he need to have made such a complex scheme work?

Is there plausible magic for increasing intelligence?

He reminds me of the Snakes and Spiders in Leiber's The Big Time. Even with time travel, humans can't change the past because time heals itself-- you need an additional set of senses? computational ability? to make the changes which will make a difference. (See also Leiber's "Try and Change the Past".)

Of course, the other answer is that Dumbledore knows he's in a genre story and is manipulating narrative logic, but that's probably not an interesting answer.