Nemesis? I mean we know what happened to the mantle, it entered Molly.. Did Nemesis also enter Molly even though it isn't apparent right now? Could this be the real reason why Mab ordered Harry to kill Molly if something happened to her? Because of Mab dies, Nemesis would come forward and take full advantage.
I think the Mantle is unalterable. Meaning that Nemesis couldn't latch on to it. So It's connection was lost when Maeve died.
I think that Mab knows that there is too much humanity left in Molly. She needs Molly to give herself to the Mantle, completely. A Winter Lady that's conflicted is tolerable, a Winter Queen is not. The Queen must be laser focused on the mission at hand. The mission to protect the Out Gates at any cost.
Remember that Molly loves Harry. Mab does not. She see's Harry as a weapon, and my guess is a disposable weapon that isn't supposed to survive past the Stars and Stones. Molly might not have what it takes to sacrifice Harry the same way Mab will do it without hesitation.