Seem like having 2 gate valves on the return line to make adjustment to the manifold can be a pain. Trying to adjust the 2 outputs to be the same would be very difficult. The concern that I have with adjusting the gate valves on the return line is the back pressure.
As for the turn over rate, just keep in mind what equipments will be running off the manifold and select a appropriate return pump for the application. Getting a lot of flow from the return pump is not that important, you can achieve it with powerhead.
maybe this???