It seems that many employees in the dealership sytem (not only MB) really detest their customers. This has been proven to me by personal experience as well as the many posts on the subject. In my company, when someone abuses a customer, he gets fired. There would be no company without the satisfied customer, but somehow this obvious axiom does not seem to apply to the auto industry. Of course there are disagreements and disputes in any business relationship, but the outright horror stories that I read about leads me to conclude that our dependence on cars in general, and certain marques in particular is partly responsible for the bad customer service we receive. For the most part, we have no alternative to the car and they know that. Therefore there is no incentive from a business standpoint to change things. We are trapped in a mess of our own making. In alot of respects it is a lot like domestic air travel--they don't really like you, and they're going to prove it to you over and over and over again. The advice to seek a good independent tech and to call AMEX to dispute those charges is good advice--you will definately breathe easier.
Best of luck