Hmmm...so either Cowl left early or he didn't see the maybe-Death Curse as an example of why Harry was making the Wardens so nervous (Perhaps Harry didn't make the Wardens nervous before GP?).
I'm don't quite get what you're saying here. Why can't Cowl be Simon and be an ally of the Fomor?
Also can you be more specific as to why re-reading of Dead Beat while specifically thinking about a Cowl = Simon theory made that theory more difficult to swallow?
1) My guess is that Cowl and Kumori were smart enough to get the heck out of Dodge once the violence kicked off. The whole point of using proxies is that you avoid taking physical risks yourself.
2) There is no definite reason that I can think of that Cowl could not be Simon as well as a servant of the Fomor.
3) A more careful re-read of Dead Beat left me with the impression that Cowl had worked with Kemmler and/or the Kemmlerites before and had a relationship with them going way back before Archangel. It is possible that this could still apply to Simon but it just doesn't quite feel right.
They treat him as a peer of sorts but not as an intense rival (like they treat each other). This makes me think that he is an ally but an outsider (not a Kemmlerite). If you look at the Kemmlerites as filling the same role as Vittorio or Bianca I think that it feels similar. An outside power offering assistance and the idiots taking the power not being smart enough to realize that they are being played.