One more piece of evidence that Cowl = SP: Cowl seems to be closely and recently acquainted with the wardens and how they feel about Dresden. That implies someone either on the Senior Council, very close to the SC, a leader of the wardens, or some combination of the three. Simon Petrovich fits all of those parameters.
There are two points that I think may make this theory less certain, however.
First, while the wizards listed in the OP are the only SC-level wizards named in the novels so far, LTW clearly indicated that there were several others who might have been tapped to the Senior Council. Whether any of those have the contacts with the Wardens necessary to say some of the things Cowl has said is unknown.
Second, I think several things suggest that Simon Petrovich's body was found at Archangel. WoJ talked about the vampires found "around Simon," seemingly destroyed by his death curse. And also if his body had NOT been found then I think that would have been noteworthy. I certainly wouldn't presume that a member of the Senior Council was dead until I saw the body (and maybe not even then), and I don't think other members of the SC would do so either. Even more important though, if Simon's body had NOT been found then one obvious possibility would be that the Reds had taken him alive. And that would be a top-priority emergency for the Senior Council, due to the possibility of Simon being turned and converted into a member of the Red Court. That would be a disaster. The fact that this never came up leads me to think that Simon's body was found at Archangel.
Now, another poster suggested that Simon/Cowl could have died and then used Corpsetaker-style body-jumping to escape. But if he did that then how did his new body sustain the serious injuries that we believe he suffered?
On balance, I think the weight of evidence is slightly in favor of the theory, but some questions do remain.