Drop the grandmother issue with your W, D7 and in your own mind. You cannot control what the grandmother does and whatever R the grandmother has with your D7 is not for you to try and control, repair or meddle in. Tell your W you don't want to discuss in any further or hear about it. You can however encourage your D7 to talk about her feelings regarding this issue with the C. That would be the healthiest way to deal with it IMO.
There is no "making up for it". Since your dad is not local when a grandparent event does come up include him, even if via webcam. Not to "make up" for your W's flaky mom but to INCLUDE the other grandparents.
It's not so much about "cutting your W off" as it is about setting firm boundaries just as she has. If she chooses not to respect the boundaries you set THEN you can end the call/talk and let her know why.
If you keep doing this on a consistent basis your W's life will become very tough but then again, that is what she wanted so let her deal with her own stuff. My H still comes to me about his mom's drinking problem. I do feel bad about it because I know what it feels like to have an alcoholic parent, it's not easy and I know it's embarrassing to talk to just anybody about it. I always say the same thing..."H, I understand how tough it can be, I hope it all works out for the best". I don't suggest HE get counseling on how to deal with it (he should), I don't suggest Al-Anon (he should be there weekly) I just validate, let him know I can relate but I do nothing more. He wanted to be on his own with his GF then let her be his "rock".
