I pulled out the OVP and tested it; no issues.
Then I removed the IACV and tested it; obvious issue - it didn't move when I applied a pulse. Clearly dead.

Luckily, my local shop with Mercedes expertise and a lot of old used Mercedes parts lying around, had a cheap replacement.

I will install it tonight, expecting it to cure the problem.

In fact, part of the prehistory is, that the engine suffered from low fluctuating RPMs and stalling a few weeks ago, prior to the present high idling.
So everything points to a IACV that was' kicking the bucket'.

/Viggo