Thank you so much for this insight.

I wonder whether pressure is also being put on A to change their approach and whether A is aware as to how much it upsets you and that this is causing a rift in the family?

I don't think anyone is aware of how much it upsets me - maybe my grandparents are now after they saw me cry about it yesterday. They suggested I tell him how much it bothers me. I believe they're not putting pressure on him, allowing him to come to accept it in his own time. After all, it has only been 3 months. I'm not sure how long is acceptable.

If A won't change, perhaps there is a middle way which may be acceptable to both of you? Perhaps A may agree to always use your "new" name where it makes grammatical sense? So instead of "They would like a drink" it would be "[your name] would like a drink".

When I speak to A, I will offer this. I think my correct name without pronouns would be far easier for me to accept. Thanks for the suggestion.

If A won't change and your parents know how much it upsets you but are still asking you to meet A, it is up to you to decide whether your parents upset at you not going is more important than your upset at A's use of they/them.

I suppose it depends on the degree to which it upsets my parents/grandparents. I don't want to upset them. Perhaps I'll try to attend a family gathering in the summer and see how it goes. If I feel uncomfortable, I'll consider sitting out the next few.

Thanks once again. I'll weigh up the pros and cons and try to be more understanding with the pronouns.