Update:
I decided to go, the JWs be damned. I came with friends, I felt defiant. This religion was not going to keep me from mourning my grandmother.
I came in, I sat next to my family on the front row. I refused to sing a song or open a bible, but I cried for my grandmother.
Surprisingly enough, it all went pretty well. I was greeted and hugged by all. My family consoled me, hugged me, cried with me, and when it was over JW or not they came by me and hugged me, kissed me, and told me how sorry they were with a few exceptions.
Yes, the "memorial service" was more about a sermon about their ideology than about my grandmother, but my second cousin gave the talk and he managed to talk a little bit more about my grandmother than probably most other memorials.
After it was over, they went to their funeral events, and I went with my friends to visit my grandparents house, take pictures, and we had a wonderful lunch remembering my grandmother in a loving fashion.
I'm not sure why it went the way it did, but I am glad there wasn't any issues. I really loved my grandmother, the JW religion or not, and I am glad I was allowed to mourn her without any unnecessary drama.