#1 is because it's a common trope. Everybody knew that one snooty rich kid in high school, it's an easy way to express a character without much work. Which, given that Draco was mostly there to help define Harry and be an antagonist when the big bad wasn't appropriate, is about the right amount of characterization for plot importance.

#2 is the nature of fanfic. Given that the Potter series started out as stories Rowling told her own kids, I think it's incredibly unlikely that she developed the characters with an eye towards internet fandom would take to them. I don't see how the behaviors of third parties can be used as an argument that A is influenced by B.