Wow! I just read this three times and love what you do with such few words. Your Umbridge is creepy and believable, her POV and voice so well done, and the McG/H references are wonderful in their sparseness. It's great how you let her imagine them so vividly that at first one almost has the feeling she's actually watching them.
Your writing, of course, is sheer beauty. No favourite lines -- I'd be quoting the whole thing back at you because it's so well-written.