Going to be bold and say ESH (90/10 split, grandpa being the 10)

I get it, grief is really hard. And not wanting a stranger replacing your grandmother at Christmas while selfish, is understandable. That shit is tough and seeing someone else that’s not her will only make it harder or worse, set you back in the healing process. And your grandpa, while I’m sure well intentioned, should consider how painful that is for everyone else.

That being said, your part of being the asshole is pretty much everything else. The part regarding the possible stroke struck me the hardest; why aren’t you being proactive and checking up on him? Why is it the girlfriends job to be a messenger for a family that clearly doesn’t like her and one that she doesn’t know very well based on how many times y’all have met. Plus just let him be happy. Who cares if she comes to the wedding, that doesn’t change the fact that your grandma is dead and the only purpose it serves is to make your grandpa feel bad.

TL;DR your grandpa should be initially more respectful of your own grieving process but you also need to grow up and accept him and his girlfriend if she makes him happy. It’s not about you at this point.