Just a thought, don't have the exact quote, and I am not even sure that Harry could undo it. I think just before Harry tries to kill Lea with a Holy Sword and loses it. Lea expresses interest in Michael's first child, and that would be Molly.. Ultimately Lea becomes "Auntie Lea" and Molly is prepared for becoming a Lady at some point. It happens in Cold Days, and in typical Mab fashion she lays the blame at Harry's feet.. In Peace Talks he apparently agrees, then in Battle Ground Mab orders him if she is killed to kill Molly.. So a boatload of guilt here for Harry to heap upon himself both earned and imagined. Not just the prospect of having to kill Molly, but saving her human mortal soul as well, would that be an incentive for Harry to go back in time to fix what he screwed up?