if the filler cap looks clean then it's pretty unlikely it's going into the engine.Oil on dipstick just looks like oil, and there's no gunk under the filler cap.
So you're saying that coolant can't make it's way into a cylinder and be burned away without evidence in the oil?
and no controls inside the vehicle would have any affect on the flow of coolant to the rear, the only way to stop coolant flow to the rear would be to clamp the heater hose lines shut.
there are no control valves or anything like that in the engine cooling system other than the thermostat attached to the water pump.
to be clear all the controls inside the vehicle do is control air flow