... It is all about their perception of what is true, not what actually is untrue, ergo a lie, but as long as they believe they are telling the truth by their lights, it isn't a lie! 
...
Note that the fae don't come anywhere close to practicing the kinds of self-deception and willful blindness that many mortals do. If the fae "believe" something is true, it's more likely to be true than if a human believes it; but it's a highly-specific, letter-of-the-law "truth" the faeries hold to. The fae are so literal-minded about "truth" and so specific in their language, that humans (who are used to dealing with "A implies B" as being virtually the same as "A means also-B") regularly get screwed-over in dealings with the fae. (q.v. "legalese," which also regularly screws-over mortals).
... However if you bargain with the Fae with the belief that they will be fair to you since they cannot lie, oops, you just got screwed... As Harry has found out trying to deal with Mab...
And the Leanansidhe!
But Harry knew better, knew the risks of bargaining with the Fae; he didn't go into those deals expecting the Fae to be "fair" in their dealings.
He just overestimated his own ability to avoid the traps; and of course was in desperate straits...
... But they did have it in hand, didn't they. Somehow Harry just happened to make his way to their cottage where he was given the unraveling...
No, the Mothers were also in dire straits, here.
Nemfection, you'll remember, is both subtle and powerful; it acts on a level on-par with the Mothers.
With Aurora having knife-in-hand and Lily on the Stone Table, the world -- and both Courts -- were one slash away from disaster. Not even the Mothers can safely plan that sort of timing, that far in advance; mortals are too random, wild, and unpredictable at the fine-detail level (even if their broader "destiny" sorts of actions largely are predictable). I suspect the Starborn are even-less "predestined" than most.
... You think it an accident that Mab shows up in his office in the early chapter telling him she just took over his contract from his godmother?
...
Lets go back to when Mab tells Harry that the Summer Knight has been murdered and the Summer Court was holding her responsible. Why in the heck would Mab go to an obscure wizard detective in Chicago to save her hide?
Not an accident, no...
But also, not an "obscure" wizard detective...
not to Mab!!!My own WAG is that Mab herself was the one who initiated the "Starbabe" plan; it was her intention, all along, to gain a Starborn-Wizard Winter Knight, and she laid her plans a generation in advance.
My Wild-Ass-Guess:
*We have seen, repeatedly, that the Big Players understand that Something Really Big is coming (they have the inside scoop on Jim's writing schedule, and the BAT is coming along quickly).
* Mab's biggest job isn't "be in charge of Winter," but "run the war against the Outsiders," and Mab is one of those Big Players. She knows there's going to be major Outsider action -- likely beyond anything she has faced before -- Really Soon. Jim has said that Mab regularly thinks in generational terms (she doesn't mind spending lives against the Outsiders, because she can so-quickly renew her forces with recruits from the Wildfae and mortal-changeling outcrosses).
* Being Mab, planning long-term, and knowing when the Starborn cycle will produce Starbabes, she looked around to find mortal-practitioner breeding stock. Lo, there was rebellious Margaret LeFay (and, I suspect, another practitioner of the "Mallory" family: Mab's hardly the "all her eggs in one basket" kinda girl! But we have zero info about Elaine's family, so that branch of the speculative tree stops there).
* Mab directed her handmaid to repeatedly cross Maggie Sr's path, "accidentally" encountering her and moving from acquaintance to familiarity, even a bit of comfort. I suspect the Ruby Waystone is partially because Mab wanted M. LeFay to be extra-comfortable with the Fae, and further divided from mortals -- such isolation breeds desperation and extreme plans.
* I think it was Mab's plotting that brought the "Starborn" facts to Maggie's attention, made the idea seem attractive. But of course with Papa Raith in the picture nothing good would come of this, so Maggie first had to get free. She knew it could cost her her life... but she "luckily" (again: no luck involved, all Mab's plan!) had this demi-friend, the Leananside, ready to become a protector / faerie-godmother to the new Starborn.
* Enter Malcolm Dresden, stage Right. NOT part of Mab's plan, but Uriel's subtle contribution. Eb testifies that he was "a man with a good soul like few I have ever seen," and Uriel saw to it that this man was Harry's father & had the rearing of Harry for the critical formative years.
* Exit Malcolm Dresden, stage Left. I suspect Lea killed him: Harry found him with a smile on his face, and cold (and WoJ is that Lea has done something that would make Harry angry enough to kill her, if he knew what she had done). But Malcolm couldn't raise a prospective Winter Knight right: he was to kind, too gentle; Malcolm (by Mab's lights) had to go.
* Orphanages are notoriously-good places to "toughen up" a child. After a few years, send him off magic tutelage by a harsh wizard, to get some conventional training & toughen him further. Perfect!, Mab would think. Note here: Ebenezer apparently knew Harry was in the orphanage, and kept a (somewhat distant) eye on him. But it was Justin DuMorne who spotted Harry's first spontaneous use of magic, and scooped in to "rescue" him.
* I think Justin's sponsor was Mab. The fae could keep an even-closer eye on Harry than any WC wizard could. Then when Justin grabbed him, Mab covered his tracks (I think the later element of Morgan "spending" his Oak-Leaf to proved impenetrable resistance to tracking was a clue-drop by Jim: as a Senior Badass, Morgan could easily have been written to do this on his own, this was Jim telling us how the Fae can stop tracking; just like Harry became un-trackable -- even to Ebenezer! -- during his apprenticeship with Justin).
So... Mab is making her plans; and they
very much involve Harry, the Starborn wizard.
The Mothers' plans are even subtler than Mab's, but I don't think they wanted to let matters get to the knife-edge they got to; "it all worked out" doesn't mean that was what they had planned, wanted, or would willingly have allowed. But, the Mothers' hands were tied in this matter; and their lips sealed, unable to give the Queens the info they needed.
... No, it took a neutral party for all the elements of the conspiracy to be rooted out, that took detective work ...
Aurora cleverly hid the truth from the Titania and from Mab. They each had to name an Emissary, to uncover the truth. Harry -- and the two Queens -- needed the "detective work." But when Harry finally worked his way up to the Mothers, it was clear they were "leading the witness," helping him to formulate the right questions to ask.
The Mothers knew the right answer, they had the solution in-hand already.
As I argue above: the situation had gotten too chancy, the issue too important. If they
could have told the Queens, they
would have.
