Cats, situations don't always turn out as we would prefer, particularly when Breeders are involved, but you can feel good about the way you handled yourself. You spoke up and told these people that they are raising a little shit, even if they didn't want to listen. And good for you for declining the invitation to teach him. I'm sure that's what Grandmoo and his ineffective parents wanted; they could farm him out to you while he continued to act like a disrespectful abusive little shit who was wasting your time and his while they gloated about their musical prodigy.
I'm sure you won't be the last in a long line of teachers who just don't understaaaaaaaaand their special snowflake. I'm hoping the little shit lets his mouth run around the wrong person and gets it slapped someday.
