I would do PVC off of the pump, ideally 1" or larger. Then a T. One side of the T has a ball valve and goes to the CO2 reactor to control flow through the reactor. The other side goes to a T OR a Y ideally. Then you have two more ball valves to control the return flow through each of those. This setup will require mass tuning, but gives the most control. Remember to use many unions in your design. I do suggest running a 6-12" section of braided tubing off of the return pump prior to all the ball valves so the vibration from the pump is not transferred. Furthermore, I suggest going large on the pump. With all these fittings, assuming you use 45s instead of 90s and some tubing if possible, you have major head height. I would go with a DC pump, this one:
(link may not work, it is the 1065gph DC Jebao return pump, $62)
30w is a small amount of power, so continuous running will be cheaper than some pumps. Also the 1000gph is fully adjustable at the pump because it is a DC motor. This will give you plenty of flow at your head height (assuming this is a below tank sump and not a basement sump). You can tune the return pump, each return and the flow through the CO2 reactor. Do note that closing the valve going to the CO2 reactor will increase the pressure to your other returns and vice versa. You'll have to adjust them in stages several times.
Ball valves work well but gate valves will give you a more true control. In the case of a return, it is a minimal advantage.