Maybe Mab isn't trying to change Harry. Harry's defining characteristic is his Will, he has overcome dozens of foes that should have beaten him, but he is still around and doing his thing. Mab is forcing Harry to make hard choices and make sacrifices, all while knowing Harry is going to fight her every step of the way. Harry is a knight worth Mab's trouble because he won't cave to Mab's desires. In Cold Days(?) Harry says something to the effect of, "You can't afford a mediocre knight" sorry don't have the exact quote at hand. Most every knight before Harry has been a mindless thug (yes, there are exceptions but not many); Harry won't get to this point because of his Will. Harry Will's himself through big bad after big bad, not because he is the strongest or the smartest in every fight but because he believes it is right and those big bads need to be put down or because it was Right. By Mab pinning Harry into corners and making him make choices, she is sharpening his Will and focusing him on her enemies and on what she thinks needs to be done. Harry will and has gone along with this because he realizes that even though Mab is evil, she is the evil humanity needs. Harry will continue to face down Mab's foes because it is right, and he will come out on top because of his strength of Will, and it's what he needs to do to protect his family, friends, and world. A tangible example of this is when Harry describes "fighting" Mother Winter in CD as a grain of sand against an ocean; the grain of sand can be pushed around but will not break. Harry will not break, he may need to be strengthened and supported, but he won't break or bend that is what Mab needs.