Not sure whose advice you are looking at, but I would try disconnecting the idle valve to see what happens. If I recall the idle valve on the 190s goes limp home at a moderate idle. If the OVP relay is involved there will be no control to the valve and there will be no fuel enrichment, usually not something that makes the engine idle high.
If the valve does different plugged in than disconnected, then it has some control and has the ability to move (they can stick). I would worry that air leaks combined with cold fuel enrichment allows the high speed. I would also go straight to the valve and monitor the signal three ways: current, duty-cycle and scope pattern.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician