Actually, the occult presented in HP is found on actual rituals, incantations, and so on. Quite well researched. Obviously not all of it, but there's quite a bit of historical accuracy. For instance, when HP and classmates transplant the mandrake roots - mandrake was used in various black magic rituals in the past. That particular scene in the book was rather disgusting, and I thought quite telling. The kids pull the plants up to transplant them, and the roots are actual little babies, living under the dirt.
No it's not. The scene in the book showed how not occult the books are, they didn't look anything like real mandrakes, they didn't discuss the real world properties of mandrake (the poison they produce), and all they did was transplant the damn things. No occult ritual, no incantation, and no so on. And certainly not well researched. The roots weren't actually little babies, they just looked like babies, creepy and wierd but hardly disgusting, and certainly not instructive in the occult.
Never heard that. It was definitely part of folklore. John Donne:
Go and catch a falling star,
Get with child a mandrake root,
Tell me where all past years are,
Or who cleft the devils foot;
I recall the footnotes about folklore, nothing about black magic.