The answer I'm giving is the correct one: for large numbers of middle-class English (and more broadly White) people, it was a retreat to a cozy power fantasy as compensation for alienation in a world in which the English (and more generally White people) were losing power as a nation/as nations, becoming demographically marginalized, personally powerless, hated by their governments, and irrelevant - it happened at a cusp moment, as one thing (English/UK/US, etc., power, traditions and sense of destiny) was finally fading out, and another (increasing neoliberalism/globalism, with the wearisome "anomalies" it brings) was on the rise.
A retreat to a cozy fantasy - I could buy that. But a retreat to a cozy POWER fantasy? I am not seeing it. To me it calls for Occam's razor.
In the first book Lee Jordan was black (or at least I assumed so, because he had dreads, which is typical hairstyle for black people). Angelina Johnson had hairstyle that looked like "worms coming out of her head" (meaning dreads, again). Cho Chang is an asian name, but she appears in the third book. So even then and there you had some (implied) racial diversity, even if not very obvious. I think your impression of "white power" comes mostly from the fact that some a lot of characters weren't really described in depth (or the other way around - were pretty much obvious and encouraged to think that other characters are also white, because otherwise they would've been described as different more clearly), but this should suffice to prove that Hogwarts wasn't really mono-ethnic school.
You also keep implying that the political correctness weighted heavily on the mind of the people back then (before year 2000), but to me it sounds more like an anachronistic superimposing the lens of the Current Year on the past works of Rowling, when they weren't at all political (or at least not as political as they became later on). Then again, I am not British so me seeing a school for wizard dominated by white people was pretty much normal sight and not a reason to get "Ah, home, sweet home...!" feeling. So you can see why I am doubtful about this being the reason for the books' popularity (including worldwide).