I think the "real world connections" in principle are hard to oversea. But I do not think that Voldemort or Grindelwald have Hitler or one of his fellows as direct antetype.
I think they are just part of a principle of our world. Even though I agree that there are not much differences between Hitler and Voldermort (and after all we know also Grindelwald) - they are all "evil" - they are not alone on this world, in history and in our presence.
The idea of the "pureblood" was not an invention of Hitler, and unfortunately - it did not die out with him.

To the other connections mentioned above- with characters of Lord of the Rings, sorry I have read the books, and I can not see any connections between Harry Potter and Lord of the Rings. I see them in a completly different way. Both great, but different.