My only other thought is do the roots have enough room in those bins after they get to a certain size. You mentioned that they begin to crash in a certain order and the ones in the middle do the best.....could the roots be getting large enough in your bins to be getting squished or something? You mentioned the corners.....there HAS to be an answer especially since they are doing great until a certain stage. The reason I think about your roots is that your plants are showing EVERY SINGLE ISSUE EVER and there must be SOMETHING in common. A 4 and 1/2 gallon bin for several larger plants just doesn't seem very big.

sorry, just wondering out loud.....