Somehow you have introduced air into the system, and I think it is by not draining the system completely and refilling to ensure no air pockets. Your cooling system lost two gallons (about 7.5L) and you only put back one. Your system is down on coolant by quite a bit.
Also, why the Wetter? The 606 in the W210 doesn't run hot at all. Even in 40C weather with the AC blasting our E300 ran below 100C every time.
1998 C230 "Black Betty"