Axel this could be a contributor to your engine temp issues, there is a thread on 500e.com regarding the use of a belly pan.
So far I haven't found anything conclusive about the engine protection pan reducing air flow, making the engine run warmer and causing an increase in underhood temperatures.
A few individuals on the 500Ecstacy website reported that running their W124 500E without the engine pan had some serious hadling issues at high speeds.
wrong cooling Tip - 500Ecstasy.com | everybody get your ROLL on
However, the engine protection pan on the 500E is not optional equipment. It is integral to the body and part of the aerodynamic treatment. It's not comparable to the optional W126 belly pan.
I guess I'm saying that the engine protection pan can
cause a reduction in air flow. However, my issues with the cooling of the M119 engine is at low speeds (sitting in traffic, basically), so there is no air flow for the belly pan to interfere with.
I'm focused on creating more, and more efficient, air flow with bigger, more powerful fans (main and aux), a well-fitted shroud and low threshold for the fans to kick in (85 C for main fans, 92 C for aux fans).
I'll also get my exhaust manifolds and downpipes Jet-Hot coated to reduce underhood temperatures.
An electrical water pump is another idea worth pursuing. The mechanical water pump, driven by the serpentine belt, is the least effective when the engine is at idle (sitting in traffic) and when a healthy coolant flow through the radiator is needed the most.