I don't have a narcissistic mother Jovial but I do have a narcissistic d.i.l. so can only talk about narcissism from that perspective.
From what you've posted, it certainly looks as if it would be good for you to put some distance between you and your mother and the relatives who she's manipulated into making accusations.
We have been estranged from our youngest son and only GC for nearly 7 years. It's been really hard, especially in the beginning, but as time has gone on, we've been able to appreciate the peace and tranquility that not having his wife's toxic behaviour in our lives has brought.
My advice for it it's worth is as a first step, put down the 'phone the moment your mother becomes abusive and do the same with relatives who are doing the same.
silverlining has suggested writing a letter and I think that's a good idea, a letter to your mother and the relatives who are making these allegations, informing them that you wont be subjected to their behaviour any more and if and when it begins, you will put the 'phone down.
Perhaps you could do the same with visits; the moment your mother becomes abusive, leave and tell her in a letter that in future that is what you will do.
Estrangement is a huge step, one only you can decide for yourself to take or not. The fact that your children are adults and have already made the decision for themselves that they want nothing to do with their GM, means that you wont feel responsible for them not seeing her. Regardless of your mother's allegations, they're old enough to have witnessed her behaviour for themselves and have made their own choice.
It was our son who estranged us thanks to his wife's manipulation and control. He also estranged, apart from his brother, his entire family. I have no idea what, if any ramifications this may have had as the family he refuses to see loved and cherished him for 27 years before he left us.
You though are considering estranging yourself from "a cruel and selfish woman who destroyed (your) life and (your) health".
Maybe take the smaller steps first and try to make your relationship with her more palatable, if that doesn't help, you can always take another look at your relationship and see if there are any benefits to continuing with it.
I wish you well.