Elaine & Kumori, the same person? What is this theory based on?
Mostly WOJ that Kumori's identity would hurt Harry, which narrows it down quite a bit. There aren't that many women wizards Harry is personally acquainted with enough for joining the other side to hurt him. We know she's not Luccio or Warden Yoshimo, because they were wounded and out of action during the conclusion of DB. Some have speculated Faith Astor or Jenny Sells grew up to manifest wizard-level talent and he'd be guilty for not finding them before Cowl, but if Kumori was the other figure on stage with Cowl at Bianca's party in GP, the timeline for Cowl to have recruited either and have her training far enough along to bring her to field events. Barring any major longshots like time-travelling (either) Maggie or heretofore unknown sister of Kim Delaney, Elaine is the head-and-shoulders standout.
I don't think it works for her to be freely willing Kumori, because Elaine is pretty clearly counter to Cowl's purposes in WN. But it's possible Justin's mental interference caused a split personality that's only able to take control part of the time.
A few other things, like Elaine would have been in the same position of being able to guess how Simon's wards would have worked from being trained by his apprentice Justin that the Council thought Harry was (though if Simon is Cowl, that's not needed). And from the Morgan microfiction, the idea that Justin was preparing a "Destroyer" could have been his other apprentice.