Make sure you have enough volume in the sump
without the check valve. It will fail at some point - usually when you're not home. Murphy's Law.
You mention the return chamber was overfilled in your leak test photo, but it's actually at the level where it should be - it should be about 1/4 - 1/2" higher than the last baffle so that there isn't a waterfall into the return chamber. If you have a waterfall that makes the baffles a moot point.
Looks like it's time to get salty!