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.

I disagree with Mab being unable to direct the Winter Lady Mantle.  She clearly intended it to go to Sarissa, but Maeve's murder of Lilly ruined that plan.  Maeve/Nemesis didn't count on Molly and Maeve/Nemesis probably thought that if Mab dealt with Maeve that Justine/Nemesis would get the Winter Lady Mantle. 

However, I agree with you that Mab has a way to deal with the White Court Demon.  We already know that Winter can create powerful unravelings that could undo the Red Vampire curse, and that Winter can quiet the Red Vampire Hunger.  The White Court Hunger seems much more separable from the Host.

How will Harry and Lara consumate their impending marriage if Lara "eats" him?  Or is the Winter Knight a power source that could be fed upon, but not devoured like the Big Foot Scion? 

Peter Principle

Some people who are good at what they do are not suited to being promoted.  Mab apparently believes that Molly, while a good Winter Lady, is not suited to the top-job. 

I suspect that Mab would like Lara to be put into the position of Winter Queen if something goes sideways.  However, given the mechanisms of Winter, at least as far as I understand them, if something would happen to Mab, then Molly would inherit the title. 

Basically, Mab is saying that Harry should take out Molly, which would give the Queen of Winter title to Lara and then Lara could find a new Winter Lady.

Really, this is all about Merlin because Merlin has a very big Harry problem. 

Harry is individually very powerful -- a top 30 wizard in terms of raw Mega-Joules/Second.  Harry is also very young, which makes that top 30 status concerning long term. 

Harry is also Starborn (which is known to all of the Senior Council). 

Harry is also extremely well-connected with Winter, Summer and the White Court.  He also has unexplained ties to both Heaven and Hell, from Merlin's point of view.  Heaven supports him -- based on his friendly relationship with the Knights; yet, Hell works with him too. 

Harry also is The Warden, a position most recently held by ... Kemmler.  Merlin understood -- though now everyone else does too -- that Harry possesses supernatural nuclear weapons.  Ethniu and the Eye are the least of it, now. 

If that were not enough, ...

Harry has supporters among the actual war-fighters -- the Younger Wardens.  These are exemplified by Ramirez, Chandler, Yuki and Wild Bill (an actual subordinate of Harry).  The commander of the Wardens, Lucio, owes possibly her very life to Harry due to the LaFortier incident (more on this).   

Harry is also connected in the Senior Council as well, much more than a roughly 45 year old wizard should.  He has unwavering support from McCoy and LtW.  Conditional support from Rashid.  Potential support from Liberty and possibly Christos as well.  Meaning that depending on the issue, Harry has more support than Merlin (who can only rely on Mai).  Note well, that the only Senior Council members not sent to the Battle of Chicago are Merlin and Mai.  This is one reason that I think Harry might be wrong about Christos, though there are other reasons.   

By the by, Harry was the BIGGEST winner in that whole LaFortier mess.  LaFortier, Morgan and Peabody die -- none of whom are friends of Harry, really.  Morgan and LaFortier were big supporters of Merlin.  Luccio is saved and owes favors.  Further, Christos may not actually be as anti-Harry (or even anti-Harry at all) despite appearances in Changes.  Harry also appeared to be a Hero to much of the assembled council (that survived).

Then Harry completely ends the War with the Reds, seemingly dying in the immediate aftermath, only to rise again to confront the Fomor. 

So, if you are Merlin, Harry absolutely f-ing terrifies you to your core.  What, therefore, is your reasonable first step? In true British Empire fashion, you secure your power base and divide your enemies.  You can't influence Mab or the White Court or Heaven and Hell.  However, you can drive Harry apart from the supporters he has among your own people. 

You can make Ramirez and the Younger Wardens suspicious of Harry.  You can arrange things to get the Senior Council members that support Harry see how in thrall he is to Mab or the White Court or both.  The fact that McCoy, LtW and Christos -- the most combat focused of the Senior Council got injured and weren't able to vote for Harry was a nice bonus. 

Of course, Merlin also set things up a later win-win with McCoy.  Either McCoy goes rogue and refuses to take out Harry and Merlin wins by getting rid of McCoy.  OR, McCoy take out Harry and Merlin wins by removing such an unpredictable and dangerous person.

My personal theory is actually that the Whamp females are infertile.  We've never seen any mention of any children of a White Vampire Woman.  Vittorio is described once as Cheserea's son, but in several other places as his aunt. 

In any event, I think Mab sees Lara as a potential replacement for Mab.  This is why she wants Harry to take out Molly if something happens to Mab.  Molly is nowhere near ready to be Mab.  Lara, on the other hand, has shown herself capable of the ruthless pragmatism and icy cold logic that Mab sees as her mode of operation. Moreover, Lara has through skill, manipulation and personal power survived for centuries and shown herself to be the strongest of the White Court -- not a negligible accomplishment.   

Let my put my lawyer hat on for this debate:

You can own a "fee simple" meaning that you can do pretty much whatever you want subject to the laws of your nation/state/county/city.  But, that last point is important.  I own a fee simple, but I can't say ... build a Chernobyl style fission plant on the property.  Perhaps the Svartelves make a "law" (or really think a law) no guests who are wizards over 6 feet 6 inches in height -- Harry actually resembles Chernobyl in many ways. 

But, you can also own property that have strings attached -- easements.  The most common ones that you see in the US are utility easements.  Meaning, the gas/telephone/electric/cable company can enter onto your property to their thing.  Home Owner Associations area also a type of easement.  But, you can have others such as if your property blocks egress/entry for another property.

There are other lesser forms of ownership too, for example a "Life Estate," which is actually the most likely scenario for Molly.  Life Estate works exactly how you expect -- it's your property and you can do whatever you want with it -- except transfer it to anyone -- until your death. 

Is Murphy dead?

Sort of, but she's off the board  -- considering how long many of the people that she associated with will live -- Molly, Harry, Thomas, Marconne, Maggie, Ramirez, Lara, etc. are effectively immortal, if not actually immortal.  Even if you take Thomas and Lara out of the list, chances are that one of these people is going to live for three, four maybe five centuries. 

Yeah, the Genoskwa surviving really doesn't make sense.  The Earth Magic thing doesn't make sense either because he was killed in a cave, yes -- but specifically in the Ice Cave.  I know that Forest People are a lot tougher than humans, but he was literally pasted. 

Why wouldn't he have come for Harry by now? It's not like everyone in the weird-side does not know where Harry lives.  Harry might be the easiest wizard in the world to find.  One need only find the nearest Burger King near the Svart-elf compound.