If Mab was able to "cure" Lea of Nemesis infection, why wouldn't Mother Winter be able to "cure" Mab? Between Mab and Mother Winter, the process shouldn't take overly long (at least not as long as it did for Lea), so... what? I mean, if she were infected, I doubt it happened any later than Small Favor, when we see her using a malk as a mouthpiece for the first time.

I don't agree that it added nothing to the story at all: it was a theory that clearly originated from Maeve, and demonstrates an action that Maeve is taking to undermine Mab. Its place in the story isn't a random worry: it's deliberate malfeasance on the part of an enemy.

We also have WoJ that Mab considers the defense of the Outer Gates to be the most important thing in her life. I believe Jim says that Mab would sacrifice herself, every human & Fae if she thought it would win her the war against the Outsiders. In Cold Days, she kills her daughter because she stands in the way of that goal.

If Mab knew she were Nfected, she would remove herself immediately, whether or not Molly is fit to be Queen. I suppose it’s possible that she doesn’t know, but as others have pointed out, what would be the point of that? It’s been played out

Reasonable, of course, is in the eye of the beholder. I would say it's reaching.

To me the whole plot of Cold Days was 'is Mab mad? Or is Maeve?' (a question which had been teased since at least Proven Guilty) and the answer was clearly Maeve, and by implication clearly not Mab (since she drove the attempt to take out Maeve, and her plotting before that point was all about revenge for being sidelined by Nemesis and its allies from Summer Knight on).
And Skin Game was a now-restored Winter striking back for slights offered during its weakened phase when second-in-command-Lea, Knight and Lady were all compromised.

<heh>
But nah.

Bob's not a perv.  HARRY's Bob is a perv.

Evil Bob?  Not a perv.  Scary as fuck, but not a perv.

Butters' Bob?  Ummm.  I don't think we know enough to be sure... but I don't think we see any signs of perviness... which just is NOT pervy Bob, especially with Andi around!

I can see your point, but I disagree. To me, Nemesis infection seems a lot more like Assimilation than enthrallment. It's like the Borg. Or maybe a Denarian-type thing, wherein the Fallen overrides the bearer's consciousness and seizes the reins. The infection itself seems to have consciousness (at least, Harry spoke to it through Sith), so maybe it's closer to a Denarian Shadow, only significantly more juiced up; it seems to be able to rewrite the core of a person, to the point that they can break their own rules (Maeve can lie, Sith disobeys Mab's will, etc.) and outright possess them when the chips are down, so I don't see how that could be fixed by turning that person into a thrall. Aside from all of that, mortal magic slides right off Outsiders, so I doubt that a mortal enthralling another mortal would have any noticeable effect on someone who's infected—the only time we've seen mortal magic hit an Outsider is performed by Harry. And it took Mab years to cleanse Lea, and she wasn't able to help Maeve at all; it's doubtful Justin could've done much of anything.

And if you read Changes closely, Leah has suppressed the infection, but isn't free of it.  Make of that what you will.

I'm of the opinion that Mab is out of Nemesis's weight class.  I'm not saying she's more powerful, just that she's powerful enough to stop it's influence.