Let's look at this:

The engine runs fine.
This means the ECU/PTCM has all it needs to run the engine, so we know the ECU has inputs from the cam and crank sensors, it has valid signals from the MAFand TB, and is telling the TB what to do with the throttle as well as telling the injectors how long to fire and the plugs when to fire.

The Instrument cluster gets most of its information (like for Tach, engine temp) from the PTCM. The speedo is driven by the brake controller.

The fan controller runs the fan at the speed the ECU tells it to, if it loses contact with the ECU, it runs the fan in emergency mode - FULL SPEED.

So, what ever is happening is stopping the ECU from talking to the Fan module and, at the same time, the instrument cluster is also loosing valid data.

See, I'd just blame the IC, but the fan speeding up makes me think otherwise. THAT one clue is what has me stuck, we know that it's part of the symptoms, but it does not "add up" with the other indications.
I'd say CAN BUS C is having an issue, but that would not explain why the fan controller is going nuts. Or would it? Could that be enough for the ECU to spin the fan fast? See, the fan module and the ECU talk over a single wire data bus that is not connected to ANYTHING else.

You almost make me think the ECU is the problem. I'd call James and Josh at the Mercedes Swap Shop, they do ECU mods and such, they'd know the intricate workings of the ECU better than anyone - you have an electronics issue, no "Mechanic" can fix this, Josh can.