According to the Donald Morgan micro-fiction, "That bastard Justin DuMorne got to him before I could. From then on, we could not be sure that the child (Harry) was not molded to be a creature of Nemesis." "We" had to be The Merlin, Donald Morgan and anyone else they trusted. Perhaps it meant the entire Senior Council; who were alive at that time, or just The Merlin's closest allies on the Senior Council. It doesn't really matter. It also doesn't matter that Ebenezar disagrees with the Merlin. The Merlin, Donald Morgan and probably a significant portion of the Senior Council were afraid and are still afraid, that Harry Dresden is controlled by Nemesis, even if Harry hasn't been fully activated by Nemesis yet.
Unless Jim is playing a huge game with the readers, we know this isn't true. We also know that Elaine tried to help Justin DuMorne enthrall Harry. Elaine claimed that she was enthralled by Justin. What if it is much more simple than that and Elaine was and is "a creature of Nemesis." It also explains why Justin wanted Elaine in the first place. Why train one potential starborn when you can train two of them. Harry and Bob have both said that Elaine had more skill than Harry, just less raw power. That may have made her a better candidate for Nemesis.
I haven't found the original WoJ on what it takes to be Starborn. The birthdate is all most of us remember; and it is not just one day every 666 years, it is a period that can last a few months. Something else has to happen. I could be wrong, but I think the word I'm looking for is activate. Something has to happen to activate an individual to be more than just have the potential to be starborn.
Also, I bet the comments Jim has made about Elaine do not rule her out as being Starborn, they are just vague and do not say that Elaine is starborn and perhaps mildly suggest she isn't starborn. On top of that, we don't know what the final ingredient is that makes someone starborn, but Elaine has had contact with a Queen of Fairie, just like Harry has. It could be something a major player like one of the Queens, an Angel, Fallen Angel or even a Titan can do, to fully make someone starborn. Titania may have chosen Elaine to be Summer's starborn in case Mab failed or for other reasons, but didn't know her candidate was tainted. Lest I forget to mention this, Elaine is the main suspect; really the only suspect we currently have, for who nemfected Aurora. It certainly wasn't Lea.
From Jim's standpoint, Elaine being nemfected also explains why the idea that Elaine might be Kumori was set up. It is a writers version of a slight of hand magic trick. Create a distraction, a red herring that gets the audience looking in one direction while the real chicanery is happening elsewhere.
Elaine claims she has hidden from the Council because she doesn't trust them. It makes sense. Elaine can see how Harry has been treated, but it can also be something more than that. Nemesis doesn't want Elaine to reveal herself until the moment is right. The moment won't be right until Harry is faltering for some reason, or being hounded by the Council to the point he is ineffective or at the very moment when Harry thinks he is facing the final big boss fight one on one. Plus, keeping Elaine under wraps keeps the White Council looking in the wrong direction, looking at Harry, not knowing he is a distraction and is actually the starborn they need if the want to fight Nemesis.
Finally, Elaine might not consciously know about Nemesis. She might believe that Justin enthralled her and she came out of it when Harry killed their teacher. At the moment Justin DuMorne died, Nemesis relinquished full control of Elaine to reassess the situation and decide on it's next course of action. Eventually, Nemesis suggested Elaine go to Summer for help and gave her the means to nemfect aurora. We may find that Cowl physically gave Elaine what she needed to do the job, but Nemesis is what drove Elaine's actions.