Something I've noticed is that Eve Dallas, the POV character, regularly identifies someone as mixed-race based on visual cues only; and I think this isn't realistic. I haven't read the books, so I don't know how it's treated in this particular situation. But I don't think it's unrealistic to say that sometimes people can identify others as mixed race. Certainly not always, and probably not regularly, because of all the variation in biological and genetic processes that you mention. But I do think it's possible to a degree, especially if you move in circles where there are a fair number of mixed race people. I know that I became a lot better at identifying people who were mixed Asian and white after living in Korea and going to school with a number of people with that background. Perhaps it's analogous to being able to, say, distinguish between people of different East Asian ancestries: sometimes it's impossible, and sometimes it's strangely easy.
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I haven't read the books, so I don't know how it's treated in this particular situation. But I don't think it's unrealistic to say that sometimes people can identify others as mixed race. Certainly not always, and probably not regularly, because of all the variation in biological and genetic processes that you mention. But I do think it's possible to a degree, especially if you move in circles where there are a fair number of mixed race people. I know that I became a lot better at identifying people who were mixed Asian and white after living in Korea and going to school with a number of people with that background. Perhaps it's analogous to being able to, say, distinguish between people of different East Asian ancestries: sometimes it's impossible, and sometimes it's strangely easy.