My bet here is that Simon's "death curse" looked like a major explosion followed by collapse of his fortress, and there was no checking for his body without moving hundreds of tons of rubble.

I think the White Council would do that if they had to. If Simon were taken alive by the Red Court and turned then the damage to the Council would be incalculable. They simply had to verify that he was dead, or find him if he was not.

But there's reason to believe that this didn't happen. In Summer Knight we find a description of the aftermath of Archangel.

"The Wardens said that they couldn't be sure, but it looked like someone let the killers in past the defenses. They didn't get away unscathed. There were the remains of half a dozen nobles of the Red Court. Many of their warriors. But they killed Simon and the rest."

The only way they could get a description like this would be if they could go in and see for themselves. WoJ confirms this, talking about the effects of a death curse: "See what happened to all the vampires around Simon when they assaulted his compound immediately prior to the onstage events in Summer Knight." (emphasis added) It certainly sounds as though the effects of Simon's death curse could be seen by someone. It even suggests that his body was actually there. But if it was not then the White Council would still have to verify his death one way or another before they could sleep at night.

If they are skilled enough to fake being low-level practitioners, perhaps.

Of course. My point was that even a Warden (Justin) was able to take two apprentices and keep them secret from the White Council until they were skilled enough to fake the Council's tests (Harry never got the chance to, of course). If Justin could do it then presumably others could. So it seems likely that there are several powerful practitioners out there who are either completely unknown to the Council or who are more powerful than the Council realizes they are.

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.