If there's one thing you count on with a narcissist, if you're unfortunate enough to not be able to walk away, is their predictability DSL.
The wealth of information available is priceless and it soon becomes clear that they are all the same, all 'text book' cases. I sometimes wonder if they have any idea, that we can learn all we need to know about how to deal with them for as long as they're in our lives.
You are still very much in the game, and as long as you are you will be a positive influence on your son and a thorn in her side. From what you've said, I agree that there are signs that your son is just beginning to see things as they are, and not as she wants him to see them.
Smiling sweetly, changing the subject, making a joke will be driving her crazy and all the while your son will be seeing the mum he knows and loves, and not the one she's trying to portray you as, and make him see.
You don't sound horribly cruel, she's the one who is horribly cruel as they all are, hoping to tear your family apart if she gets the chance.
Our ES's wife hates us and holds us in utter contempt I don't hate her but I don't pity her either. I sometimes think I should, that I should feel sorry for and those like her but I don't.
I've seen what they can do, how cruel and destructive they can be. How they'll identify their victim, seek to isolate them from the rest of their family or if it's a non family member, from the rest of their group, and then do whatever it takes yo bring them down.
I was thinking about a lioness as I was typing that, but they hunt to feed their cubs and themselves, they hunt for survival. Narcissists do what they can, because they can.