Dan, your explanation of the "rich cut" is a good one.
Sleeve Wing,
I don't see how an engine at normal operating temperature can have its "bores washed with neat fuel" on inversion of the aircraft, because the fuel will be vaporised as it enters the manifold / inlet ports. Neat fuel is a liquid.
Closing the throttle reduces the pressure drop across the venturi, which would help prevent the engine pulling in excess fuel flooding from the float chamber to a greater extent.