Well, we can eliminate that step.

Pulled both fuses related to the instrument cluster, plugged them back in, no dice.

It appears after reading the service manual that a "no fuel gauge" malfunction is fixed by one of two ways... new sending unit (NOT a fun repair) or by replacing the instrument cluster. Shocking that TWO DIFFERENT instrument clusters can't read the fuel, but can read everything else perfectly fine. Even more shocking that the fuel sending unit would magically go kapütt when installing something at the other end of the vehicle.

At this point, the AMG gauges are going back in, since I originally blamed them for the fuel needle but now it appears it is NOT gauge-cluster-specific.

As to what the heck will make the gauge spring back to life, is anyone's guess.