Hi Veronique,
My suggestion is that you do not attend the service, and my reasons are:
Funeral/memorial services are conducted and attended for the benefit of those left behind, you included. It would not be helpful to you to attend, rather it is causing you a lot of stress, therefore this service is not for your benefit.
1. You said that your grandfather is the only one that cared about you growing up. That says to me that all of the people attending the service did/do not care about you like your grandfather did. So, you are not going for their benefit, but more to honor your grandfather. As a grandmother of a wonderful child who is the victim of a psychopathic parent, I would want my child to stay away from a service that was being conducted as a means for that pyschopath to get into her head again. You grandfather is gone, and does not need your presence there.
2. You know why your mother is doing this, and you feel that she has influence over the others attending, so that she can make you look bad. She will try to make you look bad whether you go, or whether you don't go. So, keep yourself out of the snare of her web, and stay away.
3. As the experts, as well as so many on this site have said, the only way to deal with a psychopath is NO CONTACT. If you don't hear her false accusations against you, then you will not have to struggle with the evil things she says.
Please, protect yourself and stay away. That is what your grandfather would want - for you to be safe. That is what I want for my granddaughter, that is how I know.
Best wishes to you, and congratulations on staying away for 2 years. Don't let her lure you back.