One of my clients' went through the same thing a couple of years ago, shortly after buying a used ML320. If I remember correctly, we had to install a complete lock set(doors, tailgate, ignition tumbler, and glove box), an All-Activity Module(AAM), and an ME-SFI(engine management) control module. Normally, it would have cost about $2500.00, but MB stepped up and picked up half of the bill. As it turns out, an MB dealer in Florida apparently mis-diagnosed the vehicle on two separate occasions, resulting in unnecessary key replacement both times. Were it not for this, two more keys would have been available.

Normally, if a dealer makes a mistake it's their problem, and MB won't take responsibility. In this case, I think they realized that the whole idea of limiting the number of keys per vehicle was just plain dumb, so they decided to help. Speak with the service manager and ask if they'll contact MB on your behalf. You've got nothing to lose by asking. Let us know what happens.