Water will flow, but much slower than the OF is rated for. It will also be quite loud as it enters the filter. When it reduces, it will trap the air, causing big "gulping" bubbles. I see no reason for the ball valve, it only reduces flow even more. For all sump/fuge designs to work, the overflow needs to have more gph flow than the return. In this situation, I would try to decrease any restrictions from the overflows. Make two seperate inlets off your flex tubing. If not for more flow, for redundancy. (safer the better, if one clogs the other will keep flowing)