I had to come back to comment after reading it a few days ago, because I just didn't have words for the wonderful mess you made of my head.
Your Umbridge is fantasticly well portrayed as scary and just utterly, utterly wrong. I think I need a bath (in the best possible way). You get across the sense of wrongness so well.
Watching Arabella trying to make sense of things with all her key memories stolen, it really did feel like the calls were coming from inside the house. (That was a truly brilliant section heading.)
One detail that really stuck with me was Umbridge's casual comment, "But there is a way to keep them so that they’ll give you pleasure without causing any inconvenience." That's run through my head repeatedly over the last several days. So scary. I'll never look at her decorative cat plates the same way again.