Park Place Mercedes Mercedes does not stand behind their product Grapevine, Texas

2006 Mercedes R350 with 63,000 miles. All recommended service was performed at the suggested mileage, including regular oil changes every 4,000 miles (synthetic oil). Engine started making ticking sound when
running.  Put in for service at Park Place Mercedes.  They replaced a pulley at a cost of $700, and did not solve ticking sound.  Mechanic informed me that ticking sound was normal. Of course I did not agree, but was unable to convince them otherwise. 2 months later, complete engine failure, without warning.  The cost of repair is $15,000 (same as value of car).  Requested assistance from Park Place Mercedes and Mercedes USA since car was out of warranty.  Park Place Mercedes denied any assistance in fear they could be held responsible for missed diagnosis 2 months earlier.  Mercedes USA offered $2,000 toward repair and since has rescinded the offer and indicated they would provide $2,000 toward the purchase of a new Mercedes. 

What to do???