from the 5 or so 190E heater cores i have seen leaking (have fixed 2) the leak was most obvious inside the car, the way the core is positioned it will usually leak inside before it will even show in the evaporator drains, have you lifted any of the floormats to look for coolant? are you and the tech sure it isn't dye from the a/c system leaking at the evaporator drains? it may also be the heater valve that is leaking, since the valve is under the winshield grill, if it leaks the coolant has to go somewhere, it will come out of the drains under the car, take a flashlight and look through the plastic grill behind the battery area at the windshiel bottom, you should see the valve there, see if there is coolant around it, if there isn't coolant inside i would suspect that first.
as to if they used the mb coolant, it is very likely they did, when i bought my 230TE it had green coolant in it, i hate the green coolant!!! in orger to get it all out i had to disconnect many hoses and completely flush the system several times to get rid of the green tint. so a typical coolant flush will not completely remove all coolant, even if you drain the radiator and block there is still enough there too keep the green color alive.