The switch on the block goes to ground when the temperature is under 17 C and this will advance the ignition timing about 20 degrees to help with warm up. At 100 C the switch on the thermostat housing will go to ground and this will also advance the ignition 20 degrees to help with cooling down a very hot engine.
The problem with running ignition timing ATDC at idle is that it will make the engine run hotter than it would if set BTDC. The main problem, as I see it, is 100 C is already too hot and any timing advance probably won't work to cool your engine off once you're at 100 C or more. If a switch could be found that would close at 90 or 95 C then it would likely work much better. All of this is dependent upon the vacuum switch over valve working which is wired as a different circuit with the speed relays. If none of this stuff is working you are stuck with poorer warm ups after starting, a loss of 20 degrees of vacuum advance for your ignition timing ( which means poor performance and a thirsty engine ) and no vacuum advance once running too hot.
At that point, either you have to fix it so that everything works, or remove it all and go to the earlier 051 ignition system found on late 230SL's, 250SL, and early 280SL's. You would need a coil, distributor, and throttle body from an earlier engine.
