Wow. This whole post makes me very sad. I'm sad about what you are going through, and sad (and mad!) at the response of your parents. You need help right now, more help than you maybe even realize. Is there anyone at your school maybe, a counselor or trusted teacher, that you can talk to? I understand you not wanting to ruin your parents' shot at adoption, but frankly, if this is their idea of parenting maybe they shouldn't be adopting anyway. I understand that you maybe feel this is something you can or should manage yourself, but eating disorders are not that easy to overcome, even for people with tons of support.
I want you to get better, and if that means seeking help from your doctor, your school, other family, then do it. Please.