Grannyscrooge, you are a full time carer to your son, you have a pain condition, this is a more demanding time of year for you. You sound frazzled and tired to me. Being a carer is demanding even without our own health issues on top of it. Who is looking after you?
You can and need to change things for yourself. The cottage is a great idea and you have a right to decide how you want to spend Christmas. If you want to spend it at home, you can tell everyone that you are having a quiet Christmas at home with your son this year. If they are disappointed, they will get over it and make their own plans.
You are allowed to have a talk to the parents ahead of visits and let them know that you would like them to stop their children going through your things. If they don't, you have the right to tell them to please not go into Gma's cupboards, or redirect them, or ask their parents to please ask their children to stop going through your cupboards. You also have the right to reduce visits.
Limits on visits and actions with your grandchildren and adult children, especially in your own home, are okay.
I wonder though, do you always feel this way or is this just a Christmas thing? If it is Christmas, make changes. You aren't obligated to do anything. It's stressful for your son who lives with you anyway, so better for you both. If it goes beyond the season, a chat with your doctor might be in order to see what support you can get to make things easier.