Lea makes a deal with Margarite LeFay to protect Harry, then while still fulfilling her given word, either allows Harry's father to be murdered or does it herself as part of a deal with Justin DuMorne. DuMorne thinks he has won a great prize but Lea has simply offloaded the drudge work of teaching Harry basic magic skills until she can find a way to dispose of Justin. A break between Justin and Harry (Did Lea have anything to do with that? Meaning, did she have another deal with whoever summoned HWWB?) allows Lea to get Harry to remove Justin for her and to top everything off, it allows Lea to make a binding deal with a young, desperate and naïve, starborn wizard Harry.
I think you give Lea too much credit for preplanning young Harry's life. I think the original mistake was made by Margaret when she made the bargain to protect her baby in the first place. The mistake was believing that the Fae would understand fully the ways of humans. Since she was human once, Mab has this understanding, but Lea was born a Fae I believe. Early in the books we discover her ways of "protecting" Harry are a bit odd, like wanting to turn him into one of her hounds for example. I don't believe she "threatened" this out of malice as Harry thought, but because of her misunderstanding of human thinking she actually thought it was a good way to protect him.
P.S. New topic on the same character. Thinking back to the dénouement of the Battle of Chicken Pizza in Changes, Ebenezer approaches Harry and Lea and asks her to leave so he can discus "family business" with Harry. Ebenezer knew who Lea was and he knew that she knew about Eb and Harry's familial relationship.
It is one thing for Ebenezer to recognize The Leanansidhe; Lea is a very powerful figure in the Winter Court and Ebenezer has been around long enough to have had some experience with the fae, but as was pointed out in Battle Ground, Eb doesn't believe in casually dropping any information about anything, especially family. Even if Eb guessed that Lea knew that Eb was Harry's grandfather, he still wouldn't have mentioned it, he might have said something like "White Council business" instead.
Yet he must have known that Lea was Harry's godmother, thus knew about Margaret and I bet his connection. He also had to be aware that one of the main reasons for Harry's success in the mission was Mab's help when no one else was willing to help him take on the Red Court. I think the reason for Eb wanting to talk to Harry out of Lea's earshot was his reassurance to Harry that even though he was now Mab's Winter Knight, he still had free will and could use his own judgement when Mab ordered him to do something he thought was wrong.
Did Ebenezer make his own deal with Lea? I'm not speculating what such a deal might have involved, but those two know each other and not just by reputation. It suggests something more is going on here and perhaps something important.
I agree that there is a lot more to the original bargain, also that it will play a critical role in future events.