How to minimize ceiling "backwash" of air circulation in an attic HVAC system
Here is a piece of the puzzle you may not have considered.  In air conditioning mode: The cooled air being expelled from the supply registers is far heavier than the room air being drawn into the return air.  As air is cooled be the A/C unit it condenses dramatically.  This cooler, heavier air will drop right to the floor and push the warmer room air up and into the return. The warmer lighter air will gravitate to the highest point in the room.  You have little or no concern of the conditioned air being drawn directly back into the return air.  Good Luck.