I've got to agree that the Cowl/Harry duel outside the bookstore cannot reliably be taken as a measure of Cowl's strength...
1. Cowl may have been previously tired/weakened.
2. Cowl may have had reasons to "pull his punches."
3. Cowl specifically wanted to test Harry, not kill him.
So we cannot judge Cowl's strength solely from that encounter.
Cowl is a Senior Council member (or maybe on the shortlist of nominees for next empty SC seat), or else a pretty-senior Warden.
1. He's just too strong for any other WC role.
2. He seems intimately aware of WC operations & filled with inside-info.
Harry caught Cowl's initial force-blast perfectly, and it STILL almost took Harry out; and although relatively-junior, Harry is still a magical heavyweight, a powerhouse wizard. Overwhelming Harry by that degree, head-on, is the mark of a wizard with comparable raw strength AND a lot of experience.
But who?
Cowl could even be Arthur Langtry (another heavyweight I haven't seen nominated). I dislike this for two reasons -- first, it combines two enemies, which takes pressure off Harry. Not JB's style, not at all! Second, we've seen the deep dislike and even hatred that the Merlin holds toward Harry, which doesn't match Cowl's more neutral/curious/amused stance toward Harry.
Cowl's use of raw-force is a closer match for Harry's own magic than any other wizard we've seen, so TT-Harry is a reasonable theory; of course, Cowl could simply have been testing Harry in Harry's own strong suit. The big downside that I see to TT-Harry being any Mover&Shaker figure (such as Cowl is) is 2-fold... 1) acting in the past throws up SO MANY chances to F'ThingsUp in spectacular ways; one little intervention, MAYBE... but ongoing plotting, taking Kumori as an apprentice & stepping into the Darkhallow, sending hordes of uberGhouls into the Raith Deeps, etc; but even moreso, 2) being Cowl would make TT-Harry one of the driving forces behind all the low-level talents being murdered by the Malvora/Skavis plot... so, NO, not TT-Harry).
Eb - We've already had Harry go through the "you betrayed me" schtick with Eb over Eb's being the Blackstaff. I don't see JB re-running that exact sequence, so I don't see Eb being Cowl. Furthermore, if the actual stick is in fact Mother Winter's Walking Stick (as I've seen theorized, but has WoJ ever unambiguously confirmed???) I don't really see Mother Winter loaning that power to anyone who'd be doing Darkhallow magic.
LaFortier - This would be another Peabody-esque figure; he's being shown as mildly incompetent, a Rampire-pawn early on, etc. A "foe" of Harry's, but mostly as The Merlin's pawn or the RC's pawn, not one who really matters as themselves; the same way Peabody was kind of a doesn't-matter figure (until it turned out that his claws were into most of the SrCouncil & most Wardens...). In the end, I come down on "not enough info for a firm opinion" on the likelihood.
Another WhiteCouncil minor character (e.g. Klaus, etc) including other Wardens - All of these fall under the same "not enough info" mantle as LaFortier.
Simon - He was apparently like Morgan, but moreso. Major heavyweight combat wizard, likely at risk of burnout & becoming cynical, going all ends-justify-means. I mean, look at what we know: already on the Senior Council & with the Warden's combat/security to hand, he created his OWN tower-fortress and his OWN cadre of combat wizards... sufficiently morally-dubious that they were called a "Brute Squad." The Cowl/Kumori "White Council has rotted from within, and will fall in short order" perspective is right up that alleyway, probably even sitting in adjacent dumpsters. This is a likely one.
Justin - OK, lots to like here! Warden, in at the final kill when Kemmler went down, ended up with EvilBob from Kemmler's lab. He'd have to have been morally-teetering (at least) even then, but grabbing EvilBob sent him over the edge. Working on Justin=Cowl, I expect that owning EvilBob was probably all the Necromantic expertise needed to get to Cowl levels of power; and being part of the Kemmler-execution puts him neatly into the "Kemmler and his disciples are insane scum" POV. Also as Harry's ex-mentor, having some curiosity (about how strong he has turned out) and an amused/dismissive attitude, fit well with how we've seen Cowl. Justin had, however poorly, spent some years in the father-figure role with Harry, so there's THAT number to play. Justin as Cowl and Kumori as Elaine... that would be REALLY hard on Harry. Do we think Jim would torment Harry that much? Oh, yeah, we think that!
The Mirror,Mirror twist: TT-Harry wouldn't condone murdering all those Skavis/Malvora talents, or risking Paradox; but Mirror-Harry very well might. So if we combined TT+MM, then Harry becomes IHMO a very very likely Cowl indeed. I expect Jim will try to have MM-Harry break ALL the Laws of Magic.