Have added my two cents on this one before but, despite originally thinking this, I think there is way too much circumstantial evidence suggesting Elaine is not Kumori.
Dresden is highly observant. He has recognized Elaine by her height, her tone, the feel of her body, her smell, and has even had a telepathic type connection to her. I'm certain he could recognize her aura as well. Magic is also inherent to the individual and we've never seen someone use two separate languages to cast spells. Dresden has encountered Kumori. Has been held back to front by her. Has sat at arm's distance to her. Has seen her cast spells.
It would be entirely against Harry Dresden as a character for him to overlook all of this. For the same reason I would believe Cowl can't be someone we've viewed directly as well.