[untamed]

If it were me...I would build the overflow box around the two bottom 1.5" holes. You need some way to skim from the surface, and the overflow box will do the trick.

You'll need both those drains to make the Herbie. (Well..that's the best way to do it) Put a gate valve under one, and put a standpipe on the other with a height just below the surface of the overflow box. Plumb the drain pipes so that they exit BELOW the water surface in the sump.

You'll have a pretty deep overflow box, so plan for quite a significant flow-back into the sump when the power goes out.

Use the top holes for the returns. If the flow isn't too large, you could put a SQWD in the returns to cycle the flow between the left return and the right return. Or, you could put those Hydor rotating nozzles on them. If you go with the SQWD idea...I can attest that they work pretty well, but plumb to make them easy to replace because they only last a year or two.