Returns should rather go over the top with proper siphon breaks. How will this design work when the power goes out? Remember one way valves are not fail safe.The holes are for the retun of the sump !
Alternative is a return box, just like a slimline overflow, same height as overflow. But with a piece of glass on top creating a 5mm slot facing forward. If power goes out, then the water level can only drop that much, no siphon break needed. Place this return so that water makes nice circular movement on top of the tank, pushing floating stuff towards to overflow.