I doubt it was Mab. Remember Harry's total fear of Lea at that point? He was afraid she was going to turn him into a hound ...
But fear is an intense motivator, a great lever to move someone to action.
And Mab is a master at manipulation; and Lea is Mab's Handmaiden...
... At the party Lea gave Bianca the Sword, and Bianca gifted Lea with the Knife that Cowl had presented to her.. Or someone fitting Cowl's description. The Knife that turned out to be infected with Nemesis. Which in turn infected Lea, infected Maeve and led to her death, and may have tried to infect Mab until she figured out what was going on.. No, it wasn't Mab that set up Harry.
Nemesis is incredibly hard to detect; I don't think Mab knew the hidden peril at Bianca's court.
Hell, I don't think
Bianca herself understood it; she was a pawn on the board of much stronger players, playing a much deeper game.
... Here is my theory, I have posted it before. When we first meet Lea, she appeared to me to be a bit power hungry ...
More than "a bit," I think!
But note that Mab
likes her that way; it's probably a major component of why she picked Lea as her "handmaiden." Lea & her ambition is a key part of Mab's own regimen to keep herself at the top of her game.
... At the party Lea gave Bianca the Sword, and Bianca gifted Lea with the Knife that Cowl had presented to her.. Or someone fitting Cowl's description ... When she got hold of the Sword, she made a bargain with Bianca, the Holy Sword, for the Knife. She thought that the Knife would help her overthrow Mab. I don't think Lea had any idea that the Knife was infected, but I do believe that Bianca knew ...
I don't think there was an overt sword-for-knife "bargain."
Remember, Bianca didn't have the knife when Lea took the sword. I think it was when Lea gained Amoracchius that Cowl let Bianca know he could provide her with a gift commensurate with the one she was about to be offered.
I don't doubt Lea was planning on getting
something from Bianca; but she may have been open to any roughly-equal-exchange (bringing the Sword entitled her to receive a
very strong gift in return! (recall that Winter Law requires Lea to expect this (or exact retribution); the Ramps will know this, and even blind-with-ambition-Bianca would think twice about earning the enmity of the Leanansidhe)), or Lea may have had something else in mind, some item from Bianca's treasury. I think the cues in that scene suggest that Lea was a bit surprised -- and more than a little pleased -- at what she got.
But frankly, I think Bianca was too low-level to be told about Nemesis, and the true nature of the curse on the blade. She just knew it as the
Athame of Morgana LeFey, an incredibly-potent magical implement; that's enough for Bianca (it was enough for Lea, when she realized she had Morgana's Athame). Nemesis was still hidden. Remember:
Ferrovax himself was there, and almost certainly would have taken exception to Nemesis or any other Outsider-influence (WoJ puts the Great Dragons as part of fine-tuning the world -- on the level of raising mountain ranges etc -- which puts them pretty firmly into the anti-Outsider camp).