I don't necessarily think that Simon isn't Cowl. But I am not entirely convinced he isn't Justin either.
Jim often says people who are prime candidates are dead. But he also actively lies to us, and he also says death is a spectrum not a line. Dead isn't the same as gone.
So as far as I am concerned, being dead doesn't rule ANYONE out.
I will play devil's advocate though, for the sake of argument.
Simon:
- It is true, we first see Cowl on stage (although unnamed and in the background) just before Simon is first mentioned (and is said to have been assassinated). But Cowl was around LONG before Summer Knight. That is made clear in Dead Beat - at least from how Corpsetaker and Grevane talk about it. Which doesn't disprove that he created a persona potentially but that theory could apply to Justin just as easily.
- Simon was the vamps expert, but both Justin and Margaret worked with Red and White Courts at least (if not the Black as well). The meeting Eb was invited to had Lord Raith, Ariana Ortega, Paolo Ortega (I think), and several others (I'd be willing to bet Mavra) and maybe a Denarian or two. Justin was also present.
- The comparison to Lea and Eb and Justin is interesting (in terms of Cowl's magical right hook). Because Harry had never dueled them in earnest and seen what they could really do. My thinking is that Dresden had no other frame of reference for a Wizard duel at that point, so used what he had. Still though, I think it is a reasonable assumption that Cowl is almost Senior Council level at least, if as strong as any of them (if not stronger). Both Grevane and Corpsetaker found it unlikely Harry would have survived a duel with him. Interestingly, from what I have seen of Eb and LtW (and other Senior Council magic) I have found Cowl to be quite under-powered and tactically foolish. He got beaten by Harry and a few Wolves, despite also having Kumori (although one could argue that they were merely retreating or choosing to not have a flat-out fight). Rashid can put people to sleep effortlessly (and I suspect kill much the same), and he and the Merlin have both stopped an army of Outsiders with a single ward. LtW fought a Naagloshii and caused it to flee. Eb has caused more destruction via spellwork that just about anyone (afaik). Martha Liberty can do near perfect veils. Merlin (Langtry) controlled a Mistfiend single handed and contained and ordered a room in microseconds. LaFortier was supposedly able to cast mass illusions. Cristos took out a Rakshasa. And Simon himself has diverted the course of rivers and burned armies, not to mention apparently wiping out hundreds of vampires and vampire nobles with his Death Curse. Whilst Cowl is deadly at the time we meet him, and strangley resistant to damage, he isn't on the same scale (from what we have seen).
- I agree something is definitely fishy about Archangel. Definitely an inside job; but that's been clear from the get go. I think the suspect pool is very small. Even Peabody wouldn't have had enough information. Justin was considered a candidate (if he hadn't been dead - they believe he could have told Harry which implies they believe Justin would have had a way in). This of course doesn't address that Elaine also is just as much of a candidate as Harry for the same reasons he is. Of course, the whole thing could be a set-up.
- In the Paranet Papers, it is clear Simon chafes at the restrictions of the White Council and flouts the rules until Eb makes it clear to him that he can no longer. Simon also causes the worst damage to human politics since the original Merlin, apparently (due to his interference in Russia during the revolution). So it tracks that he is a very viable candidate. Cowl even talks about how many chafe at the restrictions of the White Council.
Kemmler:
- Although I think this is the most unlikely candidate, it is possible that Kemmler might be playing an incognito game. But one thinks the other Necromancers would recognise him and he would want to organise them.
- Your second point is basically irrefutable. He wouldn't need Bob were he Kemmler. And why would he wait until that Halloween?
- Kemmler is definitely a bigger threat than Cowl, in terms of destruction in the short-term. Long-term though...
Justin:
- You're assuming that the duel was exactly as Harry remembers (and his mind wasn't altered). We already know that some memories of that night were incomplete. The duel also could have been a farce.
- Even if Justin was defeated, doesn't mean he didn't cheat and come back like Kemmler or Corpsetaker.
- Cowl seems awfully interested in Harry. And who says isn't manipulating Harry?
- I admit, the encounter didn't seem that familiar. But that might have been the point, too.
- My guess is that Cowl never intended to perform the Darkhallow. The pace was set by others, so he had to turn to a last resort. The way he talks about it to Harry suggests that he wouldn't have done it if he didn't have to. He truly believes it is the best & only option (rather than a mad rush for power). Which is why Justin wouldn't have used Bob's knowledge. Time is pressing now, he might have run out of options.
- You wouldn't need to be a big player necessarily to steal the kids - just well connected. He had a Walker as an ally, among others. But he was also a psychomancer (if I recall correctly). So he could have done things they just didn't expect.
Alternate/TT Harry:
- Yeah, this is maybe even less likely that Kemmler.
- The height isn't mentioned. Harry always mentions people's height if it is close or greater than his.
- The Law of Conservation of History
- He should definitely have Harry's number. Current Harry would have mopped the floor with DB Harry.
- Yeah, the time stuff doesn't match up.
Which all gets to a new WAG I have. Cowl isn't just one person. Perhaps Cowl is both Justin and Simon - each wearing the hood and filling in when necessary. But who knows? I can't really pick between Simon and Justin, they are the closest and there are both excellent arguments for and against.