Hi Customer, I've never dealt with the question of how to relate to your mother-in-law vis-a-vis your impending divorce. I think that if I were you, I'd not move forward with the divorce until after she has passed away--since you seem to be certain this will happen with 6m or less. Since I'm a life-long student of human nature, I might visit her without anyone else being around and ask her for stories about her son, your husband. I'd hope to find out stuff about him as a child that might make him more understandable to you now. Her attitude toward him in comparison to her other children could show you if he was a favorite, or a scapegoat. And you'd be giving her a great gift by listening to her. You might find out if she believes in a higher being and what she expects when she dies. Real inner contact with a higher power is a major factor in rehabilitating a Narcissist, so count HER score on the 9 characteristics, compared to his. No matter who the person is, compassion for the dying is a rewarding experience for both people.
I understand however, that you might be reluctant to get involved, because then your own compassion for her could make you more reluctant to cut off your compassion from divorcing her son after she dies. Men are designed to be able to turn such emotional states on and off, while woman are more likely to have them ON or OFF all the time. But if you did that, you'd also be giving Greg a great gift, of getting to know and thus comfort his mom. You could also ask her about how he treats her--who knows what could arise from that?
These are some preliminary thoughts. I know you're very angry and bitter toward Greg, and you don't want to get sucked in to loving HIM more, if he's not going to change. I suspect also that if you put that bitterness on the shelf for a few months, you don't really know what will come of it. well, neither do I.