OK, look at it like this, suction line come from bottom of tank, only thing making it go into the pump is gravity. The pump can pump X GPM. The centrifuge takes 1GPM if you slightly over pressure it as I do. SO now your overflow is pumping out X-1 GPM. The suction line is flowing X GPM from the tank bottom. T in the overflow and you get X-1GPM coming from overflow, and 1GPM being pulled from the tank bottom. you still get X GPM total flow. The only real difference is where you are getting suction oil from. A large portion of the oil is simply getting recirculated. It's impossible to make the pump push out more than it takes in so the looped flow would simply circulate and only pull enough oil from the tank to recover what is lost out the fuge. Only way to make any kind of over pressure is to remove the fuge from the picture, put a check valve on the tank bottom, and wait for thermal expansion to cause the oil to expand enough to cause problems. Even if you have a check valve at tank bottom, you still won't over pressure the suction as in the above example you are losing 1GPM to the fuge.
That overflow loop system is often used on veg cars as it doesn't require near as much heat input to the fuel supply going to the IP. Instead of having to heat the entire flow going to the IP, that system only heats the new oil coming in to replace what was lost going into the engine, and all excess pump flow is simply routed back to pump suction.
I suffer/benefit from a mild/moderate form of Aspergers Syndrome, so the mechanical/physical realm for me comes natural. Sorry if my way of explaining only throws you off more.