I believe with the fan theory that the reason the Dursleys were such jerks was that they had been living with a Horcrux for 11 years, Rowling said that wasn’t her idea, but she really liked it. Point is, I think she’d admit if she didn’t think of something this good.

That’s what I plan on doing if I ever write a novel: leave a lot of open ended questions and vague hints, let the fans come up with complex theories, then choose the best one and pretend it was my plan all along.