And yet, they do not as a matter of course check their highest nobles and most critical people for possession. Either they are stupid, they can't, or a risk of Nemesis infiltration is deemed acceptable when balanced against the costs of checking. Choose one. As a simple possiblity, checking may be damaging to the person being checked. The soldiers at the Gates are cannon fodder, meant to fight and die routinely. The nobles can't be easily replaced.
I can't believe what I am saying, but don't assume that Mab is stupid.
Is it only kill, actually? What about attack? Because a very simple explanation for this seeming inconsistency might be that to check a fae for corruption is to attack a fae on some level. And Mab cannot order or even ask for her daughter to be checked, as that would be attacking her.
As you said yourself, it takes Mab time to figure out that her daughter is possessed. And she very clearly doesn't do this by having Rashid or Mothers tell her. There's no, like, regular weekend visit to the Mothers' cottage where they check the Queen and the Lady. Rashid doesn't spend Christmas with Mab to check her Court and Summer Solstice enjoying Titania's hospitality.
If they have the capability, they aren't using it. This means that the ability comes at the cost greater than the benefit it provides. And we are talking about Nemesis here.
Sorry, but this sounds schizophrenic from you. You tend to oscillate wildly on Mab's capability. On one hand, you think that Mab doesn't have cannonfodderto send (or multitasking ability) to help us deal with a Wan Kuei incursion or a naagloshii, which would take her, at most, an afternoon. On the other, you think she has enough free time to organize a check on multiple of her nobles, which would take far longer. Choose, dammit. Is Mab strapped for personnel or not? Lea is still on ice, from what I remember.
And here is the continuation of my argument. I want to bring two more points forward.
First, let's play a hypothetical. Let's say we completely stonewall Mab, and don't give her a plausible explanation of how we learned the identities of the infected. Do you think Mab would just shrug and thank her good fortune? No, of course not. That's exposing herself to attack. She has to consider where the information comes from and how it was acquired. If we don't give her a good enough explanation, she'll have to find out herself. I can see two easily possible explanations (and that means Mab would see them too):
1) We were fed this information. This means we are either willing or unwilling accomplice of Nemesis and this is an Outsider plot. This means that Mab has to consider us compromised. Maybe not infected (though I am unsure how we would read to Rashid, as Infernal Exaltation seems to be sorta kinda kin to at least some Old Ones in a way), but compromised. After Mab confirms our words to be true, she'll have to think what would sacrificing these agents be worth to Nemesis. Getting Mab's favor (she'll owe us?)? Distracting her and Rashid for some critical moment (more on timing below)? Something else? She'll have to keep all those possibilities (and more) in her mind. Any favor we'll ask of her as repayment will have to be evaluated as a possible continuation of some grand Nemesis/Outsider plot. This means that any favor we get from Mab would be a poisoned fruit by default.
2) We got information somehow (we'll be discovered as a seer soon, if we haven't been already; I mean, didn't we do some crown use in front of Old Man Matthews and/or his granddaughter, both of whom were 110% guaranteed debriefed by Winter about the whole story). Mab should be able to check that we have never been in contact with Maeve or any other of the infected. Mab most likely has a pretty good guess on everything we did in the past months, actually. She almost certainly has us under surveillance. At this point it becomes important to Mab to determine the limits of our seer ability. She needs to know which of her secrets might be compromised by us. She'll almost certainly make a number of plays to learn about our Crown, testing us in many subtle ways. And if she learns how the Crown functions? We need to be able to fight her and likely Mother Winter at the same time and be able to inflict unacceptable damage when balanced against Mab preserving all her and Winter's secrets.
Maybe we learned it from the Eldest Fetch when we exalted? But then we have kept this information for a long time, and that's suspicious.
This is my first point. By not making a plausible story, we force Mab to speculate, and investigate on her own. That's dangerous and counter-productive to removing "Enemy: Winter" background we have.
Now, my second point - timing. We have delayed meeting Mab as much as we could. We are meeting her on Halloween, and she almost certainly has to assume that we know that we can kill her (or her daughter, or Mother Winter, or some other immortal) on Halloween. She has to consider - we could contact her at any time, she was explicitly listening for our call. She knows that we have planned for this meeting ahead of time - it's guaranteed that us reserving the pub was reported to her. She knows that we haven't in any way altered our plans for meeting her. She probably will learn about us meeting Lara - she'll definitely learn about the car chase.
If we knew about Nemesis infection for a prolonged period of time (the most plausible being since the moment of our exaltation, since that's the moment we were in contact with an infected), why have we delayed telling her till this moment? What do we get by telling her as late as possible, and on Halloween? If we learned this today (like we actually did), how did we do so? She'll investigate, and she certainly has access to agents that can do psychometry (reading the past of objects) and are disposable enough to risk checking around Nemesis-related topics. That checking comes too close to discovering our Crown.
This is my second point - the timing of the reveal is suspicious and actually looks very bad from the outside.
