I like how you are thinking about the problem. You are right about the volume of fuel flowing through the return, being less than the pump is pumping out of the tank.
In reality though, the quantity of fuel that is removed through the injection process, is very small in comparison to the overall volume of fuel circulating. The holes in the wall of the swirl pot allow enough fuel in, to counteract the deficiency.
Your point, does draw attention to the need for the "venturi" though. In a similar way that many "saddle tank" arrangements maintain an equal quantity in each chamber.
Good point well made!
Cheers Aidan
Dave