You are obviously ruining your holiday by overworking yourself and placing too high expectations on yourself. Before you even consider the issue of guests inviting other guests, you need to work on paring down the celebration to a level that is a reasonable workload for you.
On the guests inviting guests issue, I guess it depends upon family culture. My children always are welcome to have friends or acquaintances and sometimes strangers over, particularly those who are going through a rough patch in their lives. However, my Thanksgiving is set up buffet style on a big countertop where guests serve themselves on big Chinet plates.
If I were doing a formal type dinner, yes, uninvited guests would pose more of an issue, but personally I would rather welcome uninvited guests to a simpler feast.
Have you ever considered having your own nuclear family take a pleasure trip somewhere for Thanksgiving?
Have you ever announced that you will not be able to host but that you will be happy to bring a dish?