IIRC, Bob said that Lash occupied that part of Harry's Mind that was not used anyway. I don't know why it's being damaged (and being healed from then on) would make him more malleable.
Yeah, it's also one of those things that straight up doesn't make sense. You don't use all of your brain for conscious thought, but you do kind of need all of it for a bunch of automated processes. Like muscular function. Kuru is a brain prion that just chews holes through your brain, and it's 100% fatal. (Also caused by cannibalism, but still—holes in your noggin=bad).
Heh. Maybe those holes are where Uriel poured soulfire, restoring some nerve connections and allowing him to manipulate Harry's hand in Small Favor.