Okay, I haven’t read all the posts, but have read the OP’s ones. Gran is her dad’s mum, but strangely her Gran has 2 children, both of them daughters “My Gran’s other daughter doesn’t offer to cook so if I don’t cook for my Gran who will”!!
Maybe OP’s just trying to change a few details, but why does her Gran’s other daughter not come into the equation?? It seems like the other (or only?) daughter, should be just as responsible as the OP and her parents, for feeding Gran at Christmas. In fact as the OP’s side of the family (really the OP solely) has provided Christmas lunch for last few years, maybe it’s Grans actual daughter’s (and/or her family) turn to take on that responsibility?
The OP says about her parents “Maybe because if I didn’t cook for my gran they’d be left with no choice but to cook for her which would then mean they couldn’t go about their day as they wish to,” and ”if I didn’t cook my parents would then have to sort something themselves and plainly change their plans. Me cooking for my Gran means my parents are free to do as they please.”
I hate to break it to the OP, but her parents wouldn’t HAVE to cook for Gran. OP seems to think that it’s solely her or her parents who have the responsibility to Gran, but there is her siblings, her Aunt (and Uncle?) and cousins too. Failing that, I would expect that there is something arranged in the local area, catering Christmas lunch for those who live alone
Gran “sees all of the family staggered throughout the day.” So everyone visits Gran (the cynical me thinks, maybe to get their Christmas pressies or money?) but no one else is bothered whether she starves or not. It looks like OP doesn’t actually spend time with her Gran at Christmas, as she just drives there and back (once dinner is cooked) so that she can get her own lunch before it spoils.
Why doesn’t OP just have her Christmas lunch at home, plating up some lovely food, then go over to Gran with her family and heat it up, while spending some quality time with her? I’m sure Gran would appreciate this more than a flying visit!
I really hope it works out for you OP, you have been so kind to your dear Gran and I bet she is really grateful for what you do for her. 💐