SilverNinja, I was like your granddaughter. I believe her psychological circumstances are to blame; her mother's doing everything for her and constant upheaval in her life. She is a child and cannot take care of herself (literally, because she has so few resources) so must conform to the parental expectations. She is not allowed to take care of herself (as you have pointed out, can't even get her own diapers), even if she had the ability/knowledge to do so. She is helpless other than in being a "problem." It is the only input she has in her life.

She needs an outside advocate; the doctors have found nothing wrong with her because nothing IS wrong with her, it's very very wrong with her circumstances and the adults in her life.