I love Sara Bareilles. Absolutely relate. I'm not sure that sending a 10 yr old to her room b/c of behavior problems is the best course of action. She's talking about hurting herself or others and he's not taking that seriously? I understand the need to see your child in a certain way and deny problems but you can't bury your head in the sand with certain issues.
The school should be a big ally for you. Talk to her teachers and the counselor. Let them know that X took her off her meds this summer and they should record behavior patterns for awhile to assess whether or not she's doing as well as she did last year. School's prefer meds in general so they should be willing to watch her.
Maybe it's something you should address with her C as well. Let her know what she's saying and how X it handling it. Not that you're tattling, just concerned that you both have a uniform approach to it. She can talk to your D and let you know how serious she is.
BTW- good to hear from you. Sorry your X is still such an a$$. You'd think with the D final he'd back off a bit. Hopefully, in time, he will.