My take was that she was a Time Traveling Maggie Sr.
Hmm, now that I think about it and what Lea may have done with her, I'm wondering about your angle instead, but with Lea perhaps holding some of the strings and Cowl not knowing about it.
We have WOJ about what Harry would do to Lea if he ever found out what she did to Maggie Sr. It partly played in with Maggie Sr. not adequately phrasing the deal as she was in a hurry. So I think there were two issues with the deal Maggie Sr made, one was for herself, the other was about Harry.
Oh, no. What about a skinwalker like Goodman Grey taking her place, or going so far as essentially becoming Maggie Sr. That would mean though Kumori isn't really Maggie Sr but a doppleganger derived from Maggie Sr in part.
We know that Hell believes Maggie Sr escaped the "Dark Prince's" grasp, so Maggie Sr died and her soul passed on. Lea could have taken her ghost, fed it with Maggie Sr's blood, then animated it to take Kumori's spot. (Uhh.. now we have an undead Kumori with wizard powers)