I suspect it was already a set up.
I'm not sure whose set up it was; I suspect Mab, tho -- Lea's boss -- having Lea get Harry motivated to go to Bianca's party. I remain convinced that Mab was deeply interested in recruiting a Starborn -- Harry -- as her next WK. So Mab was both testing his mettle, and tempering him in the crucible of violence & confusion.
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. That's why he tried to kill her with Michael's Sword, which it wouldn't let him do. It fell out of his hands and she picked it up and took off with it, remember? 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.
Here is my theory, I have posted it before. When we first meet Lea, she appeared to me to be a bit power hungry. 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. Not sure how she knew of the Knife, but Cowl seemed to be merely the delivery man in this case. Also remember Harry telling Morgan in Dead Beat I believe, that the Red Court were mere cat's paws of a more powerful group. That will be who Cowl works for, though we don't know who that is yet. Bianca, as arrogant as she was and ambitious as she was, wouldn't have realized that she was being used, nor frankly did Lea.
The plan was to take out the Winter Court by stealth with the infected knife, while occupying the Red Court and the White Council with fighting a war. The guard at the Outer Gates would perhaps be down and the Outsiders could attack. Not unlike during the battle for Chicago, the Outer Gates were attacked. Hmmmm, looks like the Fomor and even Ethniu herself were only cat's paws in the end to distract with that huge battle. Thank goodness for Rashid, who kept to his post.