I don't see how a mediator will help if one party does not negotiate in good faith, feel that she is responsible to make and meet commitments and does not hold herself responsible to meet normal and expected boundaries.
Yet, I think any experienced mediator could do this. She is not hard to understand. It is just that you are cowed and afraid of her. You feel she has got all of the power, because she has taken it. Oh wow. I'm so mad at her.
The key to any agreement will be limits and boundaries and expectations and consequences, What will happen if she does not hold to them. I mean, serious consequences. Like the agreement that she will forfeit legal custody (if that's doable in your state.) Because if she does not follow through and adhere to agreements with respect to her child with you...how is it reasonable to expect that she will do so if she is solely responsible?
I would not enter mediation with her unless there are real teeth to an agreement. What would you have to gain? You are complying with all of your responsibilities. Why give her standing if she's acting so badly?
You matter here. Not just your grandson. Not just your daughter. You. That's what worries me here. She needs to begin to respect you, your life and your home, and to think about the welfare of her child. If she abuses you too badly, who ultimately is she hurting? She's hurting her child, and herself. How foolish she is, or out of control. She risks killing the goose who is laying the golden eggs.