I'm not going to pick that apart NTSH, you clearly know your stuff and my hats off to you.
Regarding your last paragraph, I'm not even a parent but I know if I had kids and they told me the nanny thinks she's a werewolf (and remember, with all the stuff she says she's got going on there's going to be strange comments from the kids on a daily basis) I know I'd be asking questions.
Your assessment is sound I'm sure, she does have a bunch of shit going on but she's turned it up to 11 for Internet arsepats.
No fucking way is she half this crazy in her dealings with normies.
Whether she's "turning it up" for the internet, or whether she's masking it irl because she knows normal people """wouldn't understand uwu""", is up for debate - and hard to settle without seeing how she behaves in person. It's a weirdly important distinction though. If she's exaggerating for internet clout, then my assessment might swing back towards BPD a bit harder, because that's the kind of attention-seeking, pity-me, look-how-special-I-am shit that cluster Bs thrive on. If she's, instead, worked out that no one else
understands her, no one else realises she can
actually speak to the dead and do magic, no one
gets it, so she has to hide her super special powers in case
they come to take her away, the only ones she can trust with the
real her are her wonderful understanding internet friends... then yeah, that fully fits with schizophrenia or some other delusional/psychotic disorder.
Also re: kids saying weird stuff: things I have heard kids say include a) repeatedly and persistently questioning their parents about a "man that was in the house" yesterday, when there had categorically been no man in the house, b) asking repeatedly and persistently about a family member that didn't fucking exist, as though the kid knew this fictional person well and had seen them last week, and c) multiple instances of insisting with absolute sincerity that they were going to grow up to be a different sex/species/whatever. None of these kids were diagnosably mentally ill; kids just say and believe some absolutely bizarre crap sometimes. You get desensitised to it pretty quickly as a parent; they grow out of it.
Admittedly, I might also query if the kids said something weird about a hired nanny - but as we talked about, if she's masking, all it takes is her saying "oh, we were playing pretend, the kids have such a great imagination!" because she has to ~protect her special secret~, and I'd be forced as the employer to shrug my shoulders and move on. (Especially since, from this email, it sounds like both parents work long hours in demanding jobs, and... my experience is that that doesn't lead to particularly attentive parenting. Alternatively, it seems like she's been nanny to these kids for a while, maybe even since they were preverbal; and child grooming can take a lot of forms, not necessarily sexual. "This is our special little secret that I can do magic, you can't tell mommy", told insistently enough to an impressionable child, can be surprisingly (and worryingly) effective. This theory meshes well with the "I'm making the kids a video telling them I'm off to become Captain America and fight nazis" comment; if she's actually doing that, and is expecting the kids to be anything other than weirded out by it, she's quite possibly been grooming them to accept crazy bullshit from her for a while.