Agreed that the scars are an important detail and probably a clue about Cowl's identity, I just don't think he's Kemmler. And recall the conversation he has with Harry, where he states, "I don't perceive myself to be mad." So he spits on Kemmler's name and calls him a madman, then later claims he doesn't think he, himself, is insane.
Only possibility I can see is Cowl not realizing that he was Kemmler; too many bodyswaps left his personality in flux until he became Cowl, and perceived Kemmler as a wholly different, and abhorrent, entity.
That thought occurred to me, it might be Cowl's way of saying, "I'm not Kemmler anymore." Also, that would explain why Cowl needed Bob's help to do the Darkhallow, while Kemmler would presumably already know how to do it. If somebody mind-blasted Kemmler at some point, so big chunks of his personality changed and his knowledge base got scrambled, that
could explain a few things. Also, like Bob, the real Kemmler might 'hiding' behind Cowl, waiting to come out again when the surface persona finishes its work.
What could mind-scramble him? Well, apparently the White Council really,
really, really killed Kemmler in 1961, several times. Even if he could come back from that, it might explain why his mind is all out of whack.
I'm not saying I think it's true, but I can't rule it out.
As for scars and the magical healing, I can think of possibilities that could prevent it, or maybe make the scars follow him from body to body. Nothing definite, just possibilities.
I think Jamie and Adam would rate the 'Cowl=Kemmler' theory as 'possible but very improbable.'
