To add to more fuel to the plot is also Mab's comment that she thought Molly was better suited to Summer.
She may have been directly/indirectly working with Titania to groom Molly for a role in the SUMMER court, Mab had just discovered her Winter Lady was infected, but no worries she has a spare daughter if she can't cure Maeve. But if the Outsiders/Enemy has gotten THAT close to Mab then the current Summer Lady has GOT to go, Lily was in no way ready for the role she would need to play if the Enemy got further 'inside' so to speak. Molly could have started to be groomed become the Summer Knight or Summer Lady at the Proven Guilty stage.
It was only after her actions in Changes, messing with Harry's mind, able to make the hard logical calls to 'get the job done' as opposed of letting her very strong emotions/feeling for Harry get in the way of it would have gotten Mab's attention. By choosing Logic over her Heart Molly showed she had the chops to be a part of the Winter Court, and Mab tailored her plans accordingly.