The monovalve is open by default, and closed by an electrical signal. A diesel by its operating parameters would and should warm up quickly, needs heat to explode the mixture, that is why they have glow plugs, to ignite the inital mixture. I dont have the shop manuals on my TD any more, but the delay somehow sticks in my mind. It could have been one of my other cars, sorry if I sent you on a chase. The thermostat idea is a rather cheap approach, why not go to the mb parts counter and ask them if the switch exists on that model.
Your temp guage should not be moving around once it gets to operating range no matter what speed , unless you are stuck in traffic and it goes up.