Many moons ago I helped plumb a 240g reef tank. The twin 1.5" drains (if I remember correctly) went into a 4" manifold and then on to the sump. I guess I don't understand your term "y'ing"... Can you explain or show a diagram?
I frequent the CPSC website and ran across this recent press release:
Aquarium Heaters Recalled by United Pet Group Due to Fire and Laceration Hazards
WASHINGTON, D.C. - The U.S. Consumer Product Safety Commission, in cooperation with the...
I would suggest something even less complicated than a FOWLR. Maybe a macro algae tank with a pair of small fish. I have always wanted to try that. Look into the Macro Algae section here. I can't imagine you would need a skimmer, UV or and sort of additives (except maybe iron). And you...
And, ion that last picture where you have the 90 going into the sump, I would suggest another Tee there. I will look for pictures to help explain, but what I found is that the Tee helps excess air to escape before entering the sump. Make the Tee tall ehough that water doesn't come out of it.