Right about the SCWD. It would work in a small area perhaps but its too short a blast to carry very far so far as current and movement goes. You would have to have additional circulation devices to drive a wave and current through the tank. The way it works, they would be better working one behind the other, not opposing each other.
If you feed the pump from the sump, then that is NOT a closed loop. It will add that pump's volume to the overflow for the tank and sump return.