OK, that's it. I am convinced. Abbie can say the word "no." Watch in the latest video (She'll Be Back) starting from about 17:11. She says it a couple of times, and in the right context because they're telling her to go to bed and she doesn't want to.
I have heard her say it enough times now to be pretty damn sure. That said, I think that is the ONLY word she knows. But that's something, right? Why don't they discuss this at all? I wonder if they've been emotionally burned too many times with a glimmer of hope for her progress only to have it turn out to be nothing.