You are aware that the pully/blancer/hub will only go back 1 way? All the holes are not the same distance apart. One is off set so it will always go back together the same way. Sounds like you may be off 1 hole and that is why you can't get the 6th bolt back in.
On the running hot problem, I usually don't repeat myself but I have to mention again about the engine grounds. If the engine is not grounded the way Mercedes intended then it can cause gauge problems. More grounds is OK, but it still needs the original grounds. I know you are convinced that the gauge must be correct because the aux fan comes on at the correct engin temperture and they use different sensors. This is correct and generally would be a good assumption but remember they also use the same grounds. From everything I have read so far, I don't think the engine is running hot, you just believe it is.
While going down the road at 50 mph or more, you could take off the engine fan and aux fan and the car will be happy. They are only there for slow or stopped. Thermostats either work or they don't and I believe you have tried 2 or 3 with the same results. Burping these engines is easy. Just fill it up to the top (over full) and drive it. If it is too low when it cools, refill to the full mark. I just did the upper timming chain guides and flushed the cooling system while I was there and had no problems getting the system full or getting the air out.

Paul

LYL:

In checking the condition of various vacuum lines you noted that removing and reconnecting the line to the ignition unit (EZL) caused a change in engine speed; that's a good sign; at least that portion of the EZL is working.

A couple of additional checks are in order. Do you have a timing light, and an ohmmeter?

With the timing light, check the timing at idle speed, and up to about 3500 RPM. With vacuum, idle: 10-14 BTDC, 3500: 40-44 BTDC. Without vacuum, idle: 3-7, 3500: 24-28.

With the ohmmeter, check for continuity from the temp. sensor to the temp. terminal in the plug on the EZL. Which plug on the EXL do you want? If you were viewing the EZL such that the two plug connections were "down", and the vacuum connection were "up", the plug on the lower left contains the temp connection. It also contains the throttle valve switch connection, and the reference resistor connection, and an unused connection. On the dual temp. sensor the two circuits are connected diagonally, one supplying the temp gauge, and the other the EZL and the ECU. If you pull the insulation jacket back a bit (at the sensor) the green with black tracer is the wire you want.
What's all this about? If there is an open circuit between the sensor and the EZL, the advance range of the EZL will be partially inhibited, resulting in less advance than is required for proper warm-engine, cruise operation. This inhibition of advance with a cold engine (high resistance) is a design feature to speed the heating of a cold engine!!! Seems rather consistent with what you are observing. If the EZL never gets a warm signal (low resistance), the advance remains inhibited, and the engine runs too hot.

Just a thought, but do you have a temp gun? The kind with a laser dot that gives you the surface temp of what you are pointing it at. Also you can unplug the coolant temp sensors and check the ohm readings. This will get you past the potential ground issue.