Lara has always been one of those background villains, the devil you know. But definitely more worrying than even Marcone - she has better networks and has been in the game longer. Which isn't to say people should underestimate Marcone, but I think strategically she is expanding faster than he is. I think she is aware that things are about to shake up and is preparing accordingly. But I also think she is the epitome of her race, patient and dangerous but not apathetic. The appearance of being a good option, an easy ally. The White Court kill you with comfort. I always remember that line from Madeline Raith "We have pens for the kine. We call them cities". I think I am paraphrasing, but you get the point. Chilling words - using our own structures against us.
Yeah Nemesis is tricky, and either there are hard limits on how many and who it can infect or it imposes limits on itself and chooses selectively. Curiously, I always thought Titania was a good candidate for infection. She told Harry its name (which doesn't rule her - Maeve told Lily so we know that is one tactic it employs) but after that chat Harry sees the Mothers, and I feel if they were worried about that they would have implied that was the case. WOJ is that Titania will only really start having a bigger role during the BAT, which suggests she is going to make some big moves.
Just to be clear - canon is currently that Victor Sells was infected. I know that Maeve could have fed Lily misinformation, but until we get actual contradictory evidence, all we are actually doing is speculating. I am not entirely sure why we all believe that it is so unlikely that Nemesis can infect mortals/humans. Remember Leonid Kravos and the FBI agents were also in that list - which makes sense. The Gatekeeper virtually confirms this when he tells Harry that whether he meant to or not, his cases have resulted in placing thumbs in the Adversary's eye (or something to that effect).
Lily also refers to it as "a disease, a parasite, a poison...a contagion" "a spiritual malady. A mental plague. An infection spreading slowly across the earth". So she is saying it is a sentient virus of sorts. All virus' are living things so to distinguish between an infection and an entity when discussing a malevolent spiritual entity is sort of redundant. Remember Titania and the Mothers remind Harry not to say Nemesis but rather say the Adversary "lest it hear you" so it at least is somewhat sentient. It also takes full control of Cat Sith for a moment so the evidence tends to support the fact it is a sentient creature.
Lily says it "changes which ought not to change. It destroys a father's love for his family by twisting it into maniacal ambition. It distorts and corrupts the good intentions of agents of mortal law into violence and death. It erodes the sensible fear that keeps a weakly talented sorcerer from reaching out for more power, no matter the cost". All of which is true. Now we might all speculate that there is other reasons that they were corrupted. But Occam's Razor - simplest explanation is the right one.
I had not heard the theory Mab was infected...but it seems rather unlikely. And Mira, not a fact either - it certainly isn't yet canon just speculation at this point. Mab has always acted like Mab - the seeds of doubt were there true enough. But mostly Maeve was the one pointing the finger (so we know that is pretty questionable). Could Mab even stop it if she were infected? Despite all of Leah's power, she was no match. Attacks from within are the hardest to defend against. Most people Harry talked to, including the Gatekeeper and the Mothers seemed to believe being Mab was no obstacle to being infected. I always infered that Mab was hiding in that garden due to secondary events that were happening - namely she wanted to see the rescue of Molly. Which was one of her plans all along - else none of them would have ever left. Mab never just has one plan and she was setting up Molly to be a potential Winter Lady one way or another.
Winter seems to have a fundamental property that is related to pain and logic that seems to be a counter to the corrupting nature of the Outside. Almost like it was specifically built for it. I suspect that is partly why it is Winter, not Summer, that guards the Outer Gates. That cold and terrible power seems to be somewhere between an immune system, antibody and cure (within limits) to the Outsider corruption. Not necessarily Nemesis specifically, but it worked well enough. Both Winter's nature and structure is that of a defense for reality.
Kbrizzle: I am not sure I agree with the idea that the Athame carried a presence at all. I think there was a WOJ that discussed how Nemesis infects its victims and whilst Jim was tight-lipped about it, there was the indication that it was something to do with the possibility of infection. Which was implied to be an imbalance of power. So I don't think there was a sentience in the knife, rather that the power of the knife itself was tainted and allowed for an opening into the Leanansidhe. So it doesn't really need to split, rather it extends itself. Whether the Athame is still tainted or not is hard to say...although you will notice that Mab was wearing it after she imprisoned Lea so I would say not.
Bad Alias: I think you are pretty much right about magic. There is the kind of dark magic which is based in the same origins as good magic (to be so simplistic) - origins are from Inside, from Life. Then there is a kind of black magic, including Necromancy, that's source is Outsider based or at least Outside based. There definitely is a qualitative difference in the black magic that say, Corpsetaker uses, and the black magic Leonid Kravos or Victor Sells uses. Which isn't to say that one cannot use both types of black magic, and that there is no connection at all, but there is definitely a descriptive difference (that even Dresden notices). Curiously, Cowl's "normal" magic seems to be of the first kind of black magic (the source is Life). It seems he plays with both kinds.