This happened to me and we tried so hard to be nice about it and we got no where. So we started avoiding as it was way too much for my son. It’s so bloody hard as I’m like you I don’t like to make a scene. But they refuse to return him once he has had enough and the last time we visited it took two days to settle him. As hard as it is you have to do what’s best for the child.
When my LB was that age he'd just started to move around on the floor and HATED being held so that was my excuse. Whenever we went to visit any family they'd always ask but I always just used to say let me get him settled or he'll just scream at you until he's back with me and neither of us want that. Stand your ground and the ground for your child whilst they are unable to themselves. He doesn't need to get used to being passed around he's not a parcel. What he needs is to be comfortable and he will go there himself which in turn will make everyone more comfortable.
Why don't you want to confront someone who clearly doesn't care about saying unappropriate things straight in your face. I would be straight back telling her that your son doesn't need to get used to anything she had decided even if it's at HER house. Also, she would better not to force your son to do things he clearly doesn't want to. It's basic consent respect. No is no and if she can't respect you won't be exposing your son to it anymore
I'm afraid you are going to have to be confrontational. You need to set boundaries. Tell her how you feel and what you want. Make sure she understands that what you want is now what is going to happen. Tell her that your child does not have to get used to being passed around at all. You will decide what is best for your child and let her know what that is. Will your partner support you with this?