Merlin,
If I was in your shoes...I would think about drafting a letter to Ford. Detail about the incompetent mechanics, the "forgetting" to give you back your "broken" parts, and the "conversation" with the service manager. I'd also say that they value you as a customer so much that you were asked never to return. My next question would be "I'm a loyal Ford owner, being asked not to return does not put me in a good mood to buy another Ford. What can be done to keep my loyalty?"
Things will never get any better other people, if the head honcho's don't know about the crappy service some dealerships give. Plus you may get something knocked off you bill, it's at least worth a shot.
Ray