That's already been woj'd on. Summer plus winter knight in one body equals cabloowie.
The fae courts were split to create a balance between them allowing no Shadows of them to form. Mabs actually the shadow, given purpose in reality. Best metaphor I can come up with is the ying-yang and 64 trigrams. The more layers there are the harder it is to imbalance.

Cowl could easily be Kemmler then, because before he was "Bob," he was the servant of Kemmler, then Justin and evil version of himself that we saw when Harry ordered Bob to recall that version of himself... That version, clearly "Evil Bob" tried to kill Harry.

Maeve brought Peabody, there is route to outside Arctis Tor to the Phobophages lair via Pell’s theatre in Chicago. Peabody uses the same way from Edinburgh to Chicago as the Senior Council, got his photo taken, hailed a cab or got a lift from Nameless to Pell’s, opened a way to the lair, met by Maeve who smuggled him into Arctis Tor and opened the way to the Island bringing a load of giant spiders from Winter.

Nameless had been expelled from Arctis Tor following Proven Guilty, it likely added to Mab’s  deduction that Maeve was hopelessly compromised the help she gave Peabody..

This ties in with the absentee warden theory, Fortier may not have been to the island in decades, and Alfred was working to long established protocols.

If he hadn’t visited for decades this would have made it more difficult for Peabody to identify the Warden, especially if Cowl is Nameless

The empty hallow Harry lays in for his recovery, was Shagnasty's. An empty reticle in an otherwise full room seems odd til you consider where the other one went.

My theory was that Aleron Fortier was the Warden before Harry, and that his murder by Peabody was a premeditated move to sever the connection with Demonreach so that Peabody could assume the link and control of Demonreach.

Can you find the cite for that, please?  I've seen it before, several times, so I believe you.
Thing is, the citation that I know about says something else:_{-- https://wordof.jim-butcher.com/index.php/word-of-jim-woj-compilation/woj-on-harrys-mortal-allies/}

Not really, Winter is in cahoots with elements of the White Council the Gatekeeper is proof of that.

We have been given clues that the Blackstaff is Mother Winters missing walking stick, most recently in Battle Ground Eb’s shadow in wielding it is that of a Crone.

Mother Winter is an Intellectus, she knows exactly where her walking stick is at all times. Therefore Eb holds it with at least her tacit consent, which she can withdraw at any time.

In The Law Mab steps in when there is an attack on the company car, she sees it as an embarrassment, an attack upon her Knight with the Blackstaff would be seen in the same vein by Mother Winter.

Harry as the Winter Knight is specifically  Mab’s agent in the mortal world, but he is also Mother Winters.

Eb in attacking Harry didn’t use the Blackstaff.