The Mothers are pretty much unkillable, high power Immortals with Intellectus, so no one is going to be able to sneak up on them, and there are two of them, one always watching. Retirement is another matter, wearying of the role they are stuck in, wanting to relinquish it.
Mab wasn’t able to redirect the Winter Lady Mantle, like she can the Winter Knight Mantle , but she may not have wanted to, she certainly couldn’t control the Summer Lady Mantle, Maeve murdered the Summer Lady to deny Mab her choice. Retirement would allow control of the direction of the Mantles, Mother Winter uses the big circle on Demonreach so that it contains Herself, Mab and Lara but not Molly. The Circle protects reality from her extrusion into it. She retires and her Mantle goes to Mab, who accepts it displacing Mab’s Mantle which only has the choice of Lara, Molly is cut off from it by the circle and the Winter Queen Mantle cannot identify her, going instead to Lara.
The Mantles are creations, at a default they are designed to go somewhere immediately, to ensure continuity of power if they cannot go to the designated candidate they will go to the next appropriate candidate, as occurred with Molly.
If things go wrong you have Harry and Alfred on standby, they take Lara down, but release her immediately minus her demon which is kept in storage and the Mantle keeps her alive in its absence.