... she doesn't want any crap from him...

It's not a matter of "want" -- The Queenmantles are much stronger than the Knightmantles.
And Mab has been subject to one for about 1000 years.

Mab cannot help but punish Dresden's defiance; the demands of the Mantle are her ever-fixed mark.

Yes, as Queen, Mab does outrank her Knight, Harry, but when has a little thing like that stopped Harry?  No, she was giving him a firm lesson on who is the boss.

My point, though, is that I don't think the Winterqueen mantle permits Mab to allow the bearer of the Winterknight mantle to perform certain kinds of defiance; at least, not without rebuking him.

She actually likes his defiance... "finally," she says, "a knight who's worth the effort."

... Perhaps, but I believe Jim did a little retcon with Mab. In Changes I believe she explicitly says she is not cruel, but in the first books that is not the case.

Cruelty, of course, is relative.
The old joke goes "How do you punish a masochist?  You don't spank them."
The idea of "tough love" usually involves a cruel-seeming behavior.
etc.

And then, of course... we're talking MAB, the Queen of Winter.
And, infamously, "winter is cruel" (it's a stock phrase).
Except it's not -- winter is impersonal; it just is what it is.

Mab can be absolutely horrific to somebody, without it being "cruel" on her part.  Mab stands largely outside those human perspectives and moral judgements.  "Simple human kindness" is another stock phrase; but it has nothing to do with Mab!

So, as awful as early-book Mab was to Harry, she wasn't necessarily being "cruel" to him.

I'll also bring up my WAG that this is part of a multi-generational long-game plan of Mab's.  Old as she is and Guardian of the Outer Gates, she knows about Starborn, and that the cycle has come round again.  So, my WAG is that Mab laid plans -- likely beginning about a century ago -- to gain a Starborn as her Winter Knight, to combine the inherent Starborn Outsider-resistance to the Knight she has during this most-dangerous of times.  She didn't just "stumble upon" Harry as a good candidate Knight -- she arranged for Margaret to learn about Starborn phenomenon, and to become "LaFey."  She arranged for the Black Council to target Margaret as a Starbabe mother.  She arranged for Malcolm's death, and Harry's vanishing into the orphanage system, and Justin to adopt him.  Some (perhaps most or all) of this may have been executed by her Handmaiden, but the intent behind it all was Mab's.

So early-book Mab was very much in the late-stage part of her plan.  And at that point, she was all about shaping Harry; at that point she needed (or at least thought she needed) Harry to see her entirely as the Evil Queen^(tm), in fulfillment of his expectations.  What she was actually doing was crafting her new Knight.

If she had thought it would work better for Harry to think she was "kind," Mab would have acted in a kindly way, without feeling any kindness towards Harry.

I really like this idea. I feel like Jim usually has the next couple of books in his head at any time so he could definitely had started this multiverse idea back in Cold Days with the Molly car scene. Based on it and the continuity errors we've seen, I suspect that there will only be a few timelines that are dealt with or that interact with other. It'll be more like Fringe than Rick and Morty in that respect.

Jim has said that, from the beginning, he had envisioned 20 novels + BAT.
The PT/BG split pushed it up to 21, and the next book "Twelve Months" is new (not in the original plan), so he's now calling it 22 +BAT.

That's not to say he had every detail of every book solidly planned out.

For example:  Butters was originally planned as a strictly comic-relief minor character; his whole "magic Batman" personna, all the "spotlight time" as a big piece of Harry's "Scooby-Gang," becoming a KotC, and the male-fantasy-throuple with hot-werewolf-babes, that was all unplanned.

For example:  Jim didn't know, as he started Changes, which "power-up" Harry would choose when he needed it.  It was only after writing some, that he realized the WK was the one to pick; but before that Jim had considered both Lasciel's Coin & Kemmler's Dark hallow to be possible ways to write it.

Nevertheless, Harry isn't free.  Just for a current example, there's the compulsory marriage to Lara...

Nevertheless, Harry isn't free.  Just for a current example, there's little Maggie -- and Bonea -- and his responsibilities there.

Nobody is "free" in the sense of being unencumbered by any obligations to others.

Harry's obligations to Mab run a bit to the brutal side of things, it's true.  Still, it's the life Harry chose.

...  Langtry apparently really believes that warlocks always backslide, sooner or later.  From that POV, killing Molly early could save the lives and/or sanity of multiple potential victims later. 

I don't think Langtry believes that it's an "always" just "mostly" or "almost always."

But that you can never be sure, and it's not worth the risk.

It's much like the infamous "trolley problem" from moral philosophy.  Langtry is always going to throw that switch, kill the one person (who is probably warlock-bound) to save the many whom that warlock would have killed.  And if, on a (very) few occasions, that person he killed would not have gone warlock...

Well.
That's regrettable, to be sure.
VERY regretable.

But in the balance-ledger, he has saved hundreds, thousands, maybe even millions (because that's how bad a rogue warlock can get).  Arthur Langtry is OK making this choice.

...
For Harry, being enslaved by Mab is by definition bad.  The fate of Lloyd Slate alone is proof enough of that.

Harry thinks so.

But he's not actually "enslaved."  He's caught in a bargain he cannot currently escape from; but it's a bargain he substantively got to shape, to set some baselines.  He's continuously pushing back against Mab, standing up for what he thinks is right.

And we have the word of both Mother Summer and Uriel, that Harry can keep being Harry, can keep on pushing back, keep on being himself.

... I think that it is about harry and molly getting rid of the winter mantels.

I don't think so, no; for several reasons.
1/ Jim has stated that 12M will be several other things -- it'll be Chicago-centric, Harry building bonds & community (very-possibly with little-Maggie elements, e.g. education, friends/playmates, ets... maybe even concerns from Illinois' "Children & Family Services") -- it'll be about Harry's grief & trauma, losing Karrin (& everything else he lost in Changes), the psychic assaults over the years, and so on:  grieving-Harry, ptsd-Harry, stressed-parent-Harry, etc... it'll be about Harry/Lara and the WinterKnight/WhitePrincess "courtship" as they squirm on Mab's hook attempting to not-disobey-Mab (but also not have a Winter-shotgun wedding).
1a/ I still think that Mab's primary objective (in ordering the marriage) is to throw her Starborn-Wizard-WinterKnight into the Whampire Court (where Papa Raith had Outsider-summoning rituals for his porn-starlet-sorceresses to use, his own personal Outsider-fueled antimagic wards, and where (it seems) Justine got Nemfected; and let us not forget Vitto Malvora's Outsider magic) -- the Whampires look to be absolutely riddled with Outsider influence, and I think she expects Harry to improve that situation.
2/ Part of Harry's whole arc is a suite of power-ups to be a viable player in the BAT.  The  WK-mantle is a big one.  I think he'll keep it 'til the end.  Harry has no way to play in the BAT leagues without the WKmantle or similar.
3/ Keeping both Harry&Molly subject to their mantles keeps the Harry/Molly sexual-torment available as an element for Jim to include.  No narrative tension, of course, as they can't follow through... unless some sort of Whamp-y / etc element makes one or both lose control; the risk of that would be its own "narrative tension" (but I don't think Jim wants to play in such a rapey playground).  Jim's a creative boy, though, and it's still a character-tormenting element, so I don't rule something in this arena out of the picture.
4/ Kringle hinted to Harry (in Cold Days) that "masks" (by which we can reasonably infer "mantles") can be put on & taken off.  I think we'll see a LOT of that in "Fight Night" which apparently has a bunch of godlings & former-gods feeding their adulation-fixation as pro wrestlers... the whole "masks" thing is HUGE in pro wrestling
5/ There are so many deep & unresolved Winter connections to explore, and the WK-mantle keeps Harry locked into them (among other things, I'm hoping/expecting Mother Winter (at some point, not in 12M) to order her Winter Knight to "Fetch me my walking-stick, boy!" (I do not believe Harry yet realizes this is the Blackstaff artifact).