Optimum timing for the magic words "She cannot change who you are" to sink in and convince Harry what they are meant to convince him; all the time Mab and Uriel both are entirely successful lining Harry up to choose to do what they need him to do while still convinced he's making his own choices. He gets to blow steam and think he has the ability to refuse and that makes him much more pliable, compared to the state of mind he was revealed in GS to have been in during Changes, of being absolutely sure he could not affect losing himself in the evil of the Winter Knight and therefore trying to arrange his own death.
(And before this thread goes off into saying that can't be the case, any argument that Mab is prevented from manipulating the choices Harry makes runs aground on her actively preventing him from having the choice of using his blasting rod, otherwise one of his favourite combat tactics, during SmF by concealing it from his memory for much of the book. Even when Harry is directly shown that there is a hole in his mind there, he cannot get that memory back by force of will; it takes Michael praying for help for him to do so. We have yet to see any indication that Mab got any trouble for that, or that that's outside her legitimate rules of engagement.)
