Simply delightful. I love the meta-touches (the excellent "Beholder" epilogue, lines like this: It sounds like a half-arsed plot device from some cheap romance novel." [Or any number of fanfics!]) The plant details are inspired; I'm impressed by how well you manage the exposition of them: we learn exactly what they do without your having to stop the story and explain; thus we feel as if we're genuine magical readers and not Muggle outsiders.
Your characterizations of Neville and Millicent are masterful. Yes, Neville's a "hero," but he's still Neville, endearing/infuritating clumsiness and all. Yet he's basically accepting of himself, which is heartening. And I like Millicent -- blunt and rough and not apologizing for it. Particularly effective is the way that you demonstrate her beauty. You really show it instead of just telling us it exists.
Beautiful characterizations, well-paced storytelling, clever writing style: a triumph. You wield a "skillful quill," Mystery Author.