I'd get it fixed. Murphy's Law says the loud clunk and dropping of the front end will come at the most inopportune time even if you don't lose control.

Sending a copy of the NHTSA letter to MBUSA will just initiate an adversarial situtation. Why not just go back to the dealer and say you are nervous about the safety issue and ask if they can assist with the cost. If they say no, suggest you'll try another dealer. They probably don't want to lose your other service business. Most dealers will split the cost (they're not obligated to pay anything) when approached reasonably. Good luck.