I dont agree WITH migs idea... i think its over thinking it. DO NOT PUT A VALVE ON YOUR DRAIN.... PLEASE DONT! It will not do anything but cause problems. If you are using it for an isolation for work later thats one thing, but dont use it to throttle the drain. Your return pump controls draining to the sump... open it more... more water to the display more water to drain... of course this only works up to the max rating of the overflow box/bulkhead in your tank.
Secondly, the "recirc" line from the mag back to the return area will only cause bubble issues.
When you put the return lines in, it should go pump, pipe, T, then valves to the fuge and one to the main tank. That way you can control exactly how much to the fuge, and exactly how much to the display. Both will be slightly open so you wont ever dead head the pump. You wont damage the pump unless you run it for any amount of time with both valves completely shut.
Jasen