understanding my new mep802a piece by piece

I have read the applicable TM's and still need clarity on several issues. the sole function of the fuel solenoid is to open and close the fuel supply, I assume the black plastic knob that is part of the slide assembly is to provide a point to manually close the solenoid valve if the electrical solenoid fails to shut down the engine. Is that its function ? Next the adjustment of the frequency control cable which limits both low and high engine rpm according hz output is to limit wide swings in engine rpm when first started prior to manual adjustment. does this cable adjustment interact with the govenor to limit engine speed as well while electrical demand loads change

Not sure on the fuel. But i guess if you want to test it start the set and disconnect the oil pressure sensor. It should shutdown.

The cable adjusts the HZ/engine rpm. It does not sence load. Put your normal load on the generator and set with a DVOM then refrence the meter on the machine.

On the solenoid, in short, yes you are correct. it's sole purpose is basically to turn the fuel supply on and off via the fuel rack which is attached internally to the metering pumps in order to start and stop the machine.
It would be very unlikely that the solenoid itself would fail in the "ON" position, but if a malfunction was to occur, pushing the black knob left would shut the machine down as well as pulling it to the right would turn fuel flow to the injectors on, provided the electric fuel pump is running. This of course is barring any other mechanical failures within the fuel rack / metering pump / governor system.