Possible, but quite unlikely. Jim himself has said he does his best to foreshadow things, and having Cowl is TAN DAN TAAA *insert name* moment is one of those things that seem likely to happen.
Having "Cowl removed his hood and he was... generic WC member number 199 i'd seen once in a meeting but had never even mentioned on page" just doesn't cut it.
Neither does Petrovich, in that sense. To reiterate something I've said before: It tells us the Council was heavily infiltrated, but we knew that; it tells us there's sinisterness in Harry's line of mentorship, but we knew that too, twice over; it implies that whatever agenda is going on with Harry is a bit bigger than just DuMorne's plans, but we knew that too, the only thing Petrovich would add is to potenitally push the time frame back a little.
If the revelation of Cowl's identity is to be a big deal, then I can't see it just filling in the blanks of the past, not being something with implications for the future.
My own theory inasmuch as I have one, is that Cowl is a previous user/victim/ally of a force with which Harry will have to contend, or of which he will have to make use. A former holder of the Blackstaff, for example, illustrating the risk that comes from it; or another Starborn, illustrating the price of failure or success for Harry. Or has made a deal with some being Harry will have to make a deal with.
I prefer arguments that can be made in setting, though, and the big one of those for me is that unless Cowl is close to a solo operator, which does not seem to be the case, a place on the Senior Council is too valuable an asset for his faction to give up.
A fomor?
How did he summon an Outsider? (no proof but probable)
How did he know about Bob?
Why is a fan of Goethe?
Why does he uses 'Dorosh' as keyword for his magic?
Who was the traitor in Archangel?
Why Harry says he's human?
Why Lea accepted a gift from an enemy?
Is "Dorosh" a Russian word? If so, it argues against Petrovch, not for, because wizards have to use words not from their own language. (Googling it, I've found Russian and Hebrew connections for it).
The others, in order:
Fomor are like fae, yes? In which case, they can produce scions who would presumably face a Choice like changelings do. Cowl is infected, which we know allows action against a being's nature - by extension, a changeling ought to be able to choose "both" or "both, but one at a time." That would allow him to do human-only stuff, or seem human.
Can't answer about Bob, but Petrovich would only know if Justin told him - and that is only likely if Petrovich was running Justin, in which case how likely is it that Petrovich would not have taken Bob years ago?
Why not a fan of Goethe? Do we know Petrovich was a fan of Goethe?
Could still be Petrovich. Petrovich=Cowl and Petrovich=bad guy on the Black Council or whatever it really is are not synomymous assertions. Otherwise, could be anyone.
If Cowl had chosen human, Lea might not be able to peg him as a Fomor-scion.
I will say, though, that your OP is the only post I've seen arguing for Petrovich=Cowl that makes it sound even halfway plausible to me.
Poll