How many miles are on the car or better yet how many times has the oil been changed?
If the oil plug has been in and out hundreds of times sometimes you are just F'ed.
Does that mean no one ever over tightened it? Probably not. People who are changing your oil are not going to take the time to use a torque wrench on a oil plug.
If the vehicle has high miles/ lots of oil changes they will not be held liable.
You best bet is to talk to the highest ranking person at the dealership and plead your case in a nice manner.