Compelling argument.
Lol. Someone gets it.
Now as for the rest of your theory.
I have long claimed that Cowl = Justin, with Simon being a close second candidate. My mind has gone a bit squirrelly over it I think, as I vary a bit between the arguments.
I have wondered lately if Jim deliberately has left the option of who Cowl is open, with several candidates possible. Some writers do things like this to keep it interesting for themselves. Combined with Jim's tendency to make continuity errors between books it's hardly surprising we can't work it out and I
guarantee that some of us will be annoyed because contradictory evidence should rule out/highlight the actual character it is revealed to be. Assuming of course it isn't just a character that we have yet to have met.
Anyway, I don't think it can be Kemmler without a massive case of disassocitive disorder. His whole thing about being insulted to being called a Kemmler, and acting like he hardly knew him, plus referring to Kemmler's glory (like a disciple).
I would think that if Justin were to return in any meaningful way, he already has i.e. he is Cowl. While it isn't impossible he might return in another way, it would be far less impact and a bit of a waste. Normally Jim doesn't waste his moments like that.
As I have always said, Cowl stating he was wondering why Harry has the wardens worried doesn't necessarily suggest he doesn't know Harry. Certainly he knows who Harry is, that much is already apparent in that scene. It's still very possible that Justin was curious about how his student turned out...and was disappointed by Harry's apparent mediocrity. Justin is even referenced in this scene right after Cowl attacks Harry. But also consider Battle Ground and Drakul. Harry finally begins to step into his destiny and his power - Drakul even comments on it indirectly by saying he wished his own heir had such strength of will. Cowl says he wished Harry was ready for the big leagues and by BG he is. Jim even referenced the exact scene the Cowl says that in Dead Beat, when discussing how BG is "the big leagues".
I am glad someone else saw the black magic issue. Personally I think Jim just has made another error and didn't check his work. But if it is not an error, then I think Harry must have committed some other act of black magic. One example might be his assault on Justin, or even He Who Walks Behind. Perhaps it's the drawing on dark emotions. The dark compact... something that has been forgotten for a long time. I am hoping Twelve Months will perhaps examine it. TM is supposedly about how Harry deals with slot of trauma, much of which we haven't seen. I suspect some of this occurred (as it often does) in childhood. Perhaps violence, maybe even sexual abuse. Perhaps from Justin even. That would make for a dark book indeed. Perhaps the compact is part of this. Compact's are deals, and it involved ingesting human blood. Considering how messed up that is even without magic, let alone what we know about such things in the Dresden Files, surely it has affected him on a deep level. Maybe part of preparing him to be a Destroyer. Certainly to do with the Starborn stuff.
His knowledge of where Bob was (with Harry) or that Bob survived makes it even more likely he is Justin. The other Heirs of Kemmler didn't even look or care too. It doesn't rule out Simon but it does make it less likely to be anyone but them. Justin of course being Bob's rescuer...and perhaps the one who made him more manageable. The being that Bob was under Kemmler was far less controllable or sane or kind.
My guess about Evil Bob's apparent lack of knowledge about Kemmler's status is partly because Evil Bob was locked behind regular Bob's personality. Something I don't see anyone other than Justin doing. So last he knew, he was serving Kemmler. But I admit it is possible that Evil Bob was serving Justin and Harry inadvertently changed Bob. Which would suggest that Kemmler is Justin.
However... Justin's behaviour in Harry's memory and Kemmler's in the Luccio short story seem to be very different and the most compelling evidence they are different beings. That said, we know separate beings can exist in the same body.
At the very least, Cowl would have known Margaret. Like all the old players seem to.
