Your car has issues for certain.

It looks like the temperature display stays on/displaying with the key off, is this true? It should not, and it is possible that someone has crossed a switched and un-switched wire, the usual place would be a radio installation. Try unplugging the aftermarket radio if there is one. I wouldn't think there is an aftermarket alarm, so is/was there a remote start?

On the idle, disconnect the vacuum line to the shutoff valve, see if the lever stays up properly. It should not bounce and is likely causing some of your problems. There was a thread on failing shutoff valves somewhere.

My return to below idle has always been fuel filters plugging, but if that is the case then the car will have no power at higher RPM.

OVP bad will in my experience give you no tach and an ABS light (have that problem today), I've had it several times on multiple Mercedes cars. Your problem might be related, but not classic symptoms, I'd look into that glowing thing first and see if you can find the crossover between the switched and un-switched. Could even be at the power antenna? Does it behave differently with the door open vs closed?

Check the "bowden cable" running from your transmission to the throttle linkage. It should pull out easily, and return easily on its own. A starting point for its adjustment is fully-extended at full throttle.

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- Jeff