People with personality disorders (or traits of) trigger others into polar positions by their extreme behaviours.
You can see this on this thread. Either go in all guns blazing and fight fire with fire or get stuck hand wringing indecisive and confused - it’s basically the fight or freeze reaction to threat and trauma.
The best solution is to step back and step up.
No professional would go head on reacting to all of her words and actions - that’s just engaging with insanity and and giving power and credibility to her irrational behaviours.
You need to be totally indifferent to her crazy words and her smears. Who cares if she manages to convince others - that’s their issue and it’s not something you can control.
In your mind you need to imagine her as the crazy drunk loon ranting in the street - would you engage with this insanity? - would you give value to their words? Would you attempt to debate, discuss, prove another point? Would you go head to head with them?
How would you look if you did?
She is only a bit back on the spectrum than the crazy street drunk. Let her rant and rave - keep others focus trained solely on her behaviour as a crazy monologue......don’t dilute this or get in her way by engaging so that it becomes a dialogue. Let her have the stage to herself.
It’s both futile and counterproductive as you have found. It’s like wack-a-mole and will leave you reeling and exhausted and you are just adding energy / valency to the system to keep it spinning.
As PP have said this keeps the focus on you as her “target of blame” and you are inadvertently providing the tension which your DS resists in this relationship.
Drop the rope and it will all collapse in on itself. It cannot exist without your contribution to this drama triangle.
Read the BDP book recommended earlier as it will help you understand how to deal with these behaviours so that you walk this tight rope effectively and don’t leave your DS feeling abandoned - which is GF plan.
Know that the GF DP need your DS as a carer for their unstable daughter so they are never balanced.
This is like one of these videos where they are trying to rescue a stranded baby deer on a frozen lake ...... there is a delicate and strategic method that requires an informed team approach to this specifically dangerous environment.