Got a little news, guys... I can't swear to it until after I run it into town tomorrow evening, but a test run this evening with the thermostat from the 104 out of my 93 300E parts car seems to show a drastic improvement. I drove several miles at 60mph and then stopped. Temp yesterday shot above 100C in seconds. Barely touched 100 this time, and then after idling for a timed 4mins with the AC running at 72F setting when I immediately pulled off the highway to simulate a long traffic light. Less than a mile of 35-40mph brought the temp back to around 90C.

It has never come back down that quickly or crept up so slowly since the serious heat set in here in Alabama. I have been forced to shut off the AC at any traffic light to keep it under 110C and it was rare it dropped below 95C when I started movung again.

I didn't think to measure and compare the fine details of the two thermostats, but they were slightly different. The one I took out has "BTT/BTO Made in Germany" around the top of the body. The bypass plunger has "2.466 87 L6" stamped on the bottom. The plunger that closes the bypass port is 1.694" in diam and measures 1.125" from the bottom of the flange to the sealing surface of the plunger. The bypass port is 1.300" below the flange, so the plunger only moves 1.175" to close the bypass. The spring on the bypass plunger is VERY stiff. I think the thermostat is opening until the plunger seats on the bypass and then can't compress that spring enough to open further, restricting flow from the engine into the radiator.

The thermostat from the 93 had a noticeably smaller diameter plunger and it seemed shorter, so perhaps lets the thermostat open further when hot.

It will be wonderful if the solution is this simple. Will report back soon.