If you ran cold water through the hot engine while flushing the coolant, a cracked head and/or block would be possible, but I believe such cases are extremely rare on the OM606. I have a 606 car (w124) and did tons of research, this engine doesn't seem to be prone to cracking heads or even blowing head gaskets like the older w124 diesels were. Your oil cooler theory may have merit, although I know little about the oil cooling system on this motor if I'm honest, but it may be worth inspecting. I believe Hatterasguy (sorry if that spelling or name is incorrect) is knowledgable about the 606 engine, there are a few other guys here as well who know these motors much better than I. In any case, I wish you well and hope it's not a disastrous problem causing you your grief.