I think the popularity of the series is entirely down to the fact that at a certain point in the 90s traditionalism and racial pride had eventually became strengst verboten sentiments for middle-class White people.
What "traditionalism" and "racial pride" has to do with Harry Potten (as selling points)? Or do you mean that the books are british and feature white people? If anything, I would rather say that the initial lack of real-life politics was the reason behind the success of the book. Harry Potter was essentially a form of escape from that into the world full of magic and oddities.
"Mudbloods" appear in the second book. The reason why blood politics came to play was because Rowling probably realized she can't make Harry Potter fight solely against Voldemort (who is supposed to be in a very weak state), so she threw in some other "hostile" elements to spice things up. On that note, Death Eaters (as both the term and characters) appear in the fourth book, which is relatively late.
As an added hook, the series had a sort of "Hitler" equivalent [...]
I don't think that "Hitler" was really a hook for anyone. I would go the opposite direction: the elitists are so painfully stupid (with the only non-stupid examples being people who turned on them) that I doubt anyone would want to side or sympathize with them, and this is by design. This is one of the weakest points in the books, in my opinion. But an easy villain makes people (the readers) rally against him, I do agree with this much.
Rowling's mythos came at just the right time for middle-class English folks (and later the White middle class generally, globally) to be able to scratch a cosiness itch and retreat to an echo of a proud England long gone or fast disappearing, with a midwit "ironic" excuse because of the kid's book/fantasy aspect (at a time when the "irony" excuse, though already on its way out by the mid-90s, was still lingeringly valid).
There is an easier explanation: it has magic mixed with the idea that there is another world we don't notice on day-to-day basis. Magic is important, because it can be make-believe, which means unlimited potential and possibilities. The hidden world is cool, because it makes you a part of something special (you're in on a secret) and sort-of explains various anomalies and strange occurrences people can't explain normally.
The series has absolutely zero value otherwise, fantasy has been far better done by other writers. (It reminds me of those occasional club-footed attempts by "serious" authors to do s-f.)
You think Harry Potter has "absolutely zero value"? Try Twillight. Harry Potter has value as a form of entertainment and entertainment has no objective value. If people like something then it has value to them, meaning it sells, therefore gaining objective value in the process. And what makes them interested in something is very subjective.
It may be not the best (or even well done) fantasy even written, but it was written well enough to catch interest of very diverse groups of people, age-wise. Which is quite a feat. I, for one, am not surprised it works well on kids who are of school age: it was written with them in mind and Rowling's own children (if I am not mistaken) helped her write the books to appeal to them even more. What really surprises me is that even people who likely weren't target audience also got interested in Harry Potter.