M.S.
Please don't worry so much that you may be playing the turnabout is fair play game by preventing contact with the grandmother. If she is not willing for this transition to happen, she will be causing great harm to your reconciliation with your son. This will not only hurt you but your son's development emotionally. He won't grow up thinking he wished things were different for him and his grandmother - He'll grow up wishing things were better for him and you AND for him and his father.
If it takes moving away from her, do it. If it takes preventing visits, do it. Grownups with the wrong motives can easily train a child to blame and hate an absent parent. Cut off issues with DCS at the start. Make it clear as calmly as you can to DCS that this is vindictive because you have asked to have custody again. Pray that you'll be given God's favor in your dealings with DCS.
I also encourage you to pray with your son every day. Pray for your whole family together including the grandparents. This will show him that you don't hold a grudge and it will help you to be more patient and forgiving with the grandmother. It's much more difficult to harden your heart against someone for whom you pray daily. God is with you, B., trust Him.