The monovalve should have little effect on how fast the engine comes up to temperature -- as was written in an earlier post, the volume of coolant in the heater circuit is small compared to the total amount of coolant. I would say that it is either something got into the coolant circuit during replacement of the monovalve and is stuck in the thermostat or something else happened and it's just a coincidence.

The sensor and the gauge can be tested with an ohmmeter. I don't remember off hand how many ohms a good sensor is cold/hot but probably a few hundred to a few thousand -- maybe someone here has done experiments. If the gauge works at all it is unlikely to be bad -- it's just a simple milliammeter movement and either works or doesn't work at all. Replacing the sensor is easy and not very expensive and is probably the second thing to do. The first is to pull the thermostat, make sure it's OK, replace if necessary.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970