I am sorry that these events are so stressful for you, purpledaisy. Perhaps it is time to stop being a part of this social group if you really don't enjoy it.
As for the rudeness with the yard, my personality is a bit different, because I would have asked that the cars be removed, explaining that the yard could not handle the weight destruction of the cars.
The bottom line is that you only owe it to yourself to be in comfortable situations. I very seldom have guests at my home because entertaining is stressful for me as well.
Only my very good friends (quite a low number!) are allowed to enter my home. These are the ones that I wouldn't mind telling to move their cars, and they wouldn't care if I asked!
Bluemountains