I don't think you need 2 Darts for returns. 1 Dart would be plenty of flow through the sump and the other one could serve as a CL pump. Darts are not pressure pumps so use the largest pipe you can. I use all 1 1/2" Sch 26 (thin wall). While Loc-line is handy for it's ability to bend, it sure puts backpressure in the line (meaning lots of head loss).
I prefer 2, 4-way OM's over a single 8-way. Easier to plumb in my opinion. I also prefer Wavyseas to Seaswirls. Wavyseas are programmable for speed and arc while Seaswirls are not.
You could bring water to the tank using 1 Dart with a Y going to 2 WavySeas or Seaswirls that would sweep the tank from the back. Use the other Dart to an OM of your choice. I think I'd use a 4-way with outlets in the front of your tank pointed down and back.
Something like that.......