By the way, Rowling told publicly in an interview, that the entire Potter story "suddenly" was in her head within a second, whilst she drove bus or whatever. So indeed the Potter story is a true afflatus -- just no divine one.
I think there is a lot of "mythology" surrounding popular entertainment. George Lucas speaks of having a single vision, when in fact, the facts show clearly that it was merely a one off film with no major development of story, and in fact, the original trilogy has the plot and basic facts made up as they went along and changed along the way. I think the same is true of Harry Potter.
For Star Wars, originally, Luke and Leia were not siblings and Darth Vader did in fact kill Luke's father. If one watches the original trilogy, one will see that this is the case, just as the new stuff makes up things and pretends it was always like that in the story.
For Harry Potter, I only read the first two books (I think, I know I read the first) but I was around 10 years old, and at the time, I found them so poorly written, I had no desire to read further. When I was 10 years old.
Nobody who writes a single book which is so poorly written, has such a bad plot, and could only been appreciated by those who are not intellectually developed, had the entire story in one's mind before starting, and no demon would stoop that low.