I agree about the favour, something that people often dismiss.
I don't think Harry would have been able to imprison Mab without a "legal" reason. He is in Demonreach, yes, but she still owns him. Now, it would have been different if Mab has done something like trying to attack Alfred or something like that.
And humanity definitely needs Mab. Harry needs to find a way to get rid of the Mantle without harming her. I am wondering if we don't need to have a replacement for Harry before he can leave.