Hi Lee80, welcome to the group

Goodness, you have been coping with your D's BPD traits for a long, long time -- you must be exhausted.
The periodic anger-fueled cutoffs, denial of behaviors/traits, and using kids as pawns in adult conflicts, all sounds so familiar.
Do you have any alternative ways to support and contact your GD? Does your GD have a phone that her mom doesn't monitor? School email account? Is GD's dad in the picture?
It is really difficult when you want to support children of a BPD parent, but that parent wants to cut you out of the kids' lives. It can take creativity, patience, and ultimately, knowledge that even prayers, wishing the best for them, and working on yourself, is sometimes the best you can do in a given season.
My thought, wrapping up, is that your GD knows in her heart who you are and your love for her -- she's experienced it for over a decade. There may be times when GD is not strong enough to stand up to her mom, and that is OK, because she's just a kid right now, and you know that GD loves you no matter what.
Let us know how the last few days have been for you, and if we can help you continue to brainstorm ways to support your GD in the midst of her mom's behaviors.
-kells76