this allegedly witty dialogue (sorry, not a Buffy fan) that they'd been praising CC for

Well, it's wittier when it makes sense in context and isn't coming from Draco.

But that's what really makes or break it for me, when a writer is praised for a plagiarized passage; if a writer just says "Thanks!" after being praised for a line she didn't write, then it becomes very hard to construct her as doing anything other than deliberately setting out to deceive.