Thinking about this a little, if you haven't already, you should try shooting an email to big daddy corporate Mitsubishi and tell them (abridged): "I absolutely love the car, but my experience with the dealership (see attached photos) make me very unlikely to be a return customer or to recommend the brand to anyone else." Make sure to specifically "name and shame" the dealership.
You never know - maybe Mitsubishi actually cares about saving face and will slap some sense into them for you. My friend briefly worked as a car salesman and from all of his stories the sales floor is all "Rand's objectivism" until some customer eventually gets burned too hard, complains to corporate, and then an angry call comes down the line threatening them with repercussions. New Mitsu's do come with a 1-year "fit and finish" (aka spit and polish) warranty and I could see Mitsubishi going to bat for you on that one.
Dealership's position and why you will never get anything out of them:
The dealership itself will never admit any blame/reach out to Mitsu about this because it puts them in a real bad light if it's a warranty claim, and if it's not a warranty claim then they would be out $$$ so it's in their interest just to to annoy you until you go away since, really, this doesn't effect the operation of the vehicle and can be made out to be "petty" (although it isn't).