Many think Mab is creating an alternative for her Mantle in Lara rather than the current Winter Lady. There is validity to this theory, Mab has a price to pay and relatively soon if you take her conversation with Titania in BG and in her view Molly is not yet ready to be Queen.
The marriage to the Winter Knight is a conduit being created for the transfer of the Mantlebin the way she set Molly up in her training with Lea.
The ideal solution would be to effect this transfer without the marriage, certainly from Molly and Harry’s point of view, Lara? Does she realise this? Perhaps.
Lara seems rather more cold-blooded than Molly, which would surely be an asset in Mab's eyes.
My own WAG, though, is that Mab thinks the White Court hosts more Outsider-influenced agents.
There was Papa Raith; we don't know how much he was an "agent" of the Outsiders, but he had an Outsider-powered death-curse and an Outsider-powered anti-magic shield; one suspects he didn't get those without making some pretty substantive trade...
Then Justine, now wholly-possessed by Nemesis, right in the bosom of White Court power.
Maybe there was some artifact, an Athame-like item they each contacted...?
But maybe there's a working agent of Nemesis, still-unknown.
How can Mab be sure? Well, she throws a
Patented Harry Dresden Anarchygasm into the depths of the White Court, and sees what floats to the surface...
Of course, Mab
never has any multi-layered plans, to there's no chance she's
both probing for Nemesis
and grooming a potential Winter Lady <
*cough*cough*> .
But I'm tolerably sure Lara's not a virgin, so she may be ineligible to become the Lady...?
OTOH, Mab IIRC did specify that it was "becoming a mother" that was the Mantle issue (although she didn't say that "remaining a virgin" is
not an issue) ...
Unless Jim WoJ's an answer, I think we'll just have to wait and see what he writes...