Cowl has been at the game a while - long enough to have a history with the other Kemmlerite disciples. If you think he was sandbagging as Beckitt, why? That couldn't have been Cowl's actual birth identity - the cops arresting someone who seems mid-30's to 40's but has an ID with a birthdate far enough back to account for Cowl's acquaintance with Kemmler and the other heirs would have raised some eyebrows. The Beckitt identity was a nobody on the supernatural who's - who radar, so sandbagging as the assistant to an inferior like Sells seems like a lot of trouble to go to for faking the death of an alias identity that nobody cares about anyway.
And if Helen Beckitt had even a fraction of Kumori's skill and power, she hardly would have needed to mess around with feeding intel to the Denarians to hurt Marcone, after seducing her way into his confidence. She could have had her revenge directly in an effectively unlimited number of satisfyingly horrible ways.
Plus, Cowl and Kumori were swanning around Bianca's party well before the timeline for Mr. Beckitt's death and Mrs. Beckitt's parole.
If Cowl was anything to the plot of SF, it makes much more sense that he was the theorized unseen tutor who was setting Sells up with dribs and drabs of training to use him as a cat's paw. Cowl or Lord Raith (or both, if they were allies in The Circle) are the best candidates for that.
