I still don't fully understand where going on. Is the return section being emptied by the pump then a rush of water fills it again? I haven't read your build so I'm not really sure what's happening on the top side of the pipes.
I was assuming the two drain lines one would be the drain and the other is an emergency. If that's the case then the single line from the container to the sump should suffice as long as it's far enough under the water to pull full siphon. I'm just guessing though. I think one of the easiest things would be to restrict your return line so both drains can keep up.
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