Hey there, Marie,
You are really in the thick of it, huh?
Welcome to this forum. There is a lot of support, other people's experience, and guidance through this dark place you're in.
The thing that jumps out at me most - other than just (as noday so eloquently put it) Holy Drama, Batman! - is the idea you've suggested that setting boundaries won't or didn't work.
You have only begun to set boundaries. It is indeed a process, and the longer and firmer you stick to yours, the more effective it will be.
Think of a flood pushing up against a wall that was constructed to keep the water away. The water will push and push, the force becomes greater, and then eventually it recedes. If it had a mind, it will have understood that there's no way it's gettin through that wall.
You guys definitely do need some distance. Most recovering alcoholics will say that they knew their loved ones did still love and care, but that they needed to be away. No matter what she says (threats, drama, manipulations) don't worry that she will think Scott or you don't care for her.
I think that it will be challenging for Scott to truly get away, and you two guys take care of your relationship during this stressful and possibly conflict-filled time.
The grandmother's garbage is just another manipulator in your life. Ignore it, she doesn't know any better.
Peace...