adding to your things to think about:
if the dealer possesses an "Factory Authorized MB Dealer" status and this came from the used car side (Starmark program in effect- but not applicable on this particular vehicle), there may be some documented "reps and warranties" generally stated i.e. customer satisfaction, used car reconditioning process, quality, vehicle history checked out...etc.- - these types of buzz words/ phrases are what you seek (in writing, in ads, signs on the premises). I recall a "list of MB client rights" brochure/ website?
...if you do have luck finding these (the more the better), your attorney may effectively ghost-write a friendly but focused letter referencing these "reps and warranties" while requesting specific remedies based upon applicable law and/or based upon your preferences note: a ghost-written correspondence is usually quite effective while allowing a stronger option to be implemented in future correspondence ie one sent by your attorney.
...at this point, barring any indication to the contrary, I would assume that the dealership will desire to rectify this unfortunate circumstance as quickly and amiably as possible & to your satisfaction...
no real need to immediately bring out big guns, send nasty grams, commence the smear campaign, threaten legal action, etc. as these paths may be more "situationally appropriate" in stalemate negotiations