Hey Worn,
I'm really sorry for your loss. I lost my grandmother six years ago (she was also 92), and had to deal with a very similar situation. I stayed away, and lived with terrible guilt afterward. I felt like I had betrayed her. Fortunately, I had a good therapist at the time who helped me to realize that my grandmother would have wanted what was best for me. It's okay to mourn privately, in your own way. Funerals are for the living, not for the person who has passed, and in your case, the people who are still living will cause you extreme discomfort. It sounds like your grandparents always had your back (as mine did). Your grandfather loved you very much and would no doubt understand your decision to avoid an uncomfortable situation. You don't need to be at that funeral to show him that you cared. He knows.
Please do NOT feel guilty. Your uncle, your cousin, and your NM should be feeling guilty for the things they've done, but you've done nothing wrong. That's what Ns do to us. They mistreat us, and make US feel like we're at fault. Grieving a loved one is a very personal thing. Don't let them ruin it for you. You've made the right decision. I think that when you are able to visit your grandfather's gravesite, and be alone with him, that you'll find some inner peace. Take care of yourself and to heck with that entire dysfunctional lot.
Kathy