I mean we all know the real reason she won't fuck with Maggie- It's not practical. Maggie primary (and currently sole) potential use is as leverage against her father ...
Mab has already fucked with Maggie, and is likely to keep doing it.
She took Maggie's dad to be her Winter Knight. That is fucked up in
soooo many ways, right there.
She has Maggie's oldest sister as Winter Lady. You know, the
cool sister, who went all goth-y and rebellious, but made up afterwards, and still comes home sometimes and does snowball-fights and other cool stuff. But in the end, Molly's still the Winter Lady, and is gonna be icy in all sorts of subtle ways.
And she's slipping Maggie the kinds of gifts that every little girl dreams of... the kinds of gifts that only a Faerie Queen can provide. And weasels her fucked-up way into Maggie's higher estimation.
... Mab:
A) Has no need to play that Card, because she has Harry by the short hairs and she know it ...
Mab won't
always have Harry by the short hairs (and I suspect she knows it). But she'll have SOME sort of a handle on Harry so long as she has input on Maggie.
B) is equally aware that if she plays that Card, she's going to have to either kill Harry (and cost herself a useful agent for no benefit), or risk throwing down with a Necromantic God after Harry decides "Y'know what, fuck you Mab, I'm gonna go eat a Capital City and then come back to finish this conversation".
Pretty sure that's a threat that Harry cannot make good upon. Until he figures out how to un-Mantle, there's nowhere he can go that Mab cannot know, cannot find him. The Mantle inside him is a part of Winter, part of Mab's own power. The moment Harry starts Necromanting, Mab is gonna feel it, and stop it (unless she figures she can control even Darkhallow-Harry via the Mantle). He can
maybe get a moment of privacy if he draws a circle... but probably not! With Winter already inside him, I will assert that Mab herself is inside the circle, too!
You recall near the end of Cold Days, he figured out he could step across the big circle on Demonreach because he was PART of the island, in some sense already inside it. Confirmed(ish) later by Molly repeating his logic with mud from the island itself. Winter inside Harry means a mere circle will no longer keep out Mab!
Last but not least: I suspect the Darkhallow is gone from his mind, from when Lash took the psychic bullet (it remains to be seen whether Bonea knows some/all of Lash's info) and so much of the Lash-knowledge (Ghouls' speach, etc) departed.