LW2 - Say "we've decided, and it's not up for discussion." If she presses, then "yikes look at the time I gotta go". Not "here are our reasons." If you give reasons, then she'll take that as an invitation to give her reasons too, and suddenly you're arguing about whose reasons matter more.

In general, any goal of getting another adult to agree with you or see your perspective is likely to keep frustrating you forever, because it depends on someone else changing their mind. Instead try a goal of "gram and I don't argue about the wedding anymore", which is something you can get that doesn't depend on her.

Also don't worry about other family members who hear her complain. She's not convincing them or causing them to be rude to you, right? If it's out of your control, and it doesn't affect how they treat you, then no point adding it to your worries.