It sounds too obvious but try changing the cold start injector valve. Its easy but expensive. The only one I could find was a through an MB dealer and cost $173(Aust). Its a Bosch part so you could try a bosch dealer for a better deal.

Its located right on top of the front of the engine has a fuel line from the injectors and a wiring connection. Two allen bolts and its done, mine was leaking. You could try seeing if it works by pulling it off the manifold, leaving it connected to the fuel line and starting the car from cold. If fuel sprays out it works I guess, although it should shut of when the engine is warm. Obviously be carefull atomised fuel can put your cigarette out.

There is also a supplementary air valve that supplys additional air during cold starting. You can take this off and test it. It should be fully open at -20c and fully closed at +65c. Its pretty close to the starting valve and has a hose in from the air cleaner and out to the manifold. The Sensor sits in the coolant line.

John