For Simons dead I trust what is said about it in Summer Knight and the woj about his dead curse. Simon as traitor would not have left the white council, he was in too good a position to do loads of damage. Or, if he was behind the fall of Archangel he could have delivered the fortress without too much struggle and arranged for wizards captives to be turned by the reds. In stead we see dead curses employed.
Cowl is clearly hiding his identity that means he has an identity worth hiding. With his strength in mortal magic, he can apparently summon outsiders, and Harry's feel about his magic I think he is human.his conversations sound human as well.
He was behind Lea's infection and the coup in white night. Both actions For Nemesis. His conversation shows some imbalance that I used to attribute to black magic using but nemesis possession fits the bill as well.
The WoJ doesn't actually say it was his death curse. It could easily have been a member of the Brute Squad who realized that Simon was Cowl. Not only that but we may also have a precedent in surviving death curses in Harry's pyrofeugo in GP. Cowl may have had Kumori ressurect him after using his death curse (to make it look more realistic to wizard examiners).
Simon is an SC level wizard so he is probably a high ranking member of the Circle and/or a person infected by Nemesis that Nemesis places a high priority on. How much damage could Simon have really done? He was a well known figure in the wizarding community and close friends with Eb and Martha Liberty (and information expert). Not only that but he was far far away from the WC headquarters.
Peabody did far more damage than Simon could ever have done.
Not only that but "dying" did tons of bad stuff to the WC and left him free to pursue the Darkhallow. It makes sense that Nemesis/the Circle would send such a powerfull operative to become a literal GOD. They of course wanted to ensure the best possible chance that they could get the prize.
If he had delvered the fortress without much struggle than the WC would have gotten suspicous. Some of the top wizards in the world are in it and they would have gotten suspicous if things weren't completely decimated. Also, the WC would have suspected something was up if there weren't any bodies. Why were there no death curses is what they would have been thinking. Also, how could he have arranged for wizard captives?
I'm not seeing how anything else in your post clashes with Simon being Cowl
. In fact, don't they strengthen it?