I will start off by saying that I am no expert on these engines, but I do know that if a repair was done a few months ago, and that repair has failed, that the dealer is trying to make you responsible for the problem. I won't go as far as to saying that it wasn't installed properly, since I have no foundation for a statement like that, however, if there is a secondary problem that caused its failure, why didn't the dealer notice that problem the first time. I don't think that it is reasonable to need to replace head gaskets as often as you would change oil. If they did the gasket before you purchased the car and then again after your purchase, and now it is bad again, I would say that either they are not doing a good job at the repair or there is a systemic problem with the car that needs to be found before the gasket is replaced again. Unfortunately, you are at the mercy of the dealer that you have brought it to, but on the other hand, you can also take it to another dealership for a second opinion. If that opinion differs from the first, then you might have a claim. If it is the same and they say that something else failed causing the now blown head gasket problem, then you will just have to bite the bullet and spend the money to repair it. I would dig deeper into the history of the car to learn if this has been an ongoing problem and when it first started. Many times, present problems have a history and that is why the past owner sold/traded the vehicle. They were tired of paying to fix a perpetual problem.
As much as I hate going to the MB dealerships, sometimes they are the only ones with the experience to diagnosis and repair a problem such as this. Many times spending more is like spending less in the long run....