I was rereading where Ethniu had just found out that Michael, Maggie, and the others had not been killed. What I found interesting (and Harry did not seem to notice)
The Titan's living eye widened
"Listen betrayed me"
Then Harry said "Sure, that's the takeaway here"
Harry seemed to think that Listen's mistake was considered a betrayal, but I suspect Dresden is wrong. Listen had known the people in the home weren't real, and had actually betrayed Ethniu.
So if my theory is true, that Listen had betrayed Ethniu, can you think of why? Does this giveaway the identity? Could Listen be Elaine, the Starborn working for Ethniu?
Tbh I think you might be taking Harry's statement a bit literally.
It read to me more of his usual snark; it felt like sarcasm.
As in, "Sure Ethniu, you see a minion/lover/agent of yours failing to achieve his objective as a 'betrayal' when he was actually just outsmarted by my friend, and you are focussed on that small detail when the actual point is that you were so arrogant to not understand that you lost before you started, you chose to be dramatic and destructive and lost because of your lack of understanding and clarity"
Or something like that.
Remember, Harry makes his statement in the context of telling her that she has been tricked by faeries and he is condescending to her. He is trolling her. In the broader context as well I think it's also Harry realising these big, ancient, powerful beings are actually so limited by their strengths - Harry faces them all down and beats them by guile, skill, will and no small amount of luck. They should win by rights but they don't. Ethniu is dangerous, but in many more ways Harry is more dangerous.
Of course, if Harry had access to godlike power...that might be a really bad time. Let alone Marcone or Kincaid (are we sure he isn't a starborn...seems much like Listen, almost certainly a scion of Drakul).
You could be right though and Harry really was trying to let Ethniu know she was betrayed by Listen. In either case, Listen might well have been working for other bosses, or might have his own agenda or both.