17 year old wants to leave the home of an emotional abusive mother and live with grand mother. Mother is threatening legal action.

Wait untilk you turn 18 and there is nothing she can say or do to stop you. It would take longer than that to fight to be allowed in absense of her permission. Not to mention the cost.

Agreed. Until you reach the age of majority (18)m you are in your mother's custody and she can make trouble for grandma if she takes you in. But if you are 17 then it will be less than a year and you can live where you want.