in your post you never mentioned water loss nor did you mention what type coolant you used. and from your description of turning the heater on to cool down i am assuming your cooling system is performing ok.
1. on aluminum using the Pgreen brand is not the best. you need something specifically made for aluminum. if however u did use the Pgreen brand you might be able to get a radiator shop to use a solvent to try and clean out any corrosion. of course that has other negative issues. talk it over with that vendor and see if they can "Guaranty" against problems. maybe MBZ dealer can help with factory approved stuff.
2. on the new rad did it have the same number of cross flow tubes as the mbz brand. is the shroud still intact. are the fins of the rad clogged up or bent over restricting air flow.
3. i'd go ahead and replace the thermostat 25$ is cheap insurance. but before i threw the old one out i'd heat a pan of water and drop it in. if it doesn't move much or at all toss it. if it moves fully open / fully closed it's a keeper save it for next time.
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