You might be nitting picks there. Bass is not that directional and reflections aren't that straight forward. I've hung membrane traps in the ceiling corners, like angled soffits. Placement is not as critical as you might think for this type of trap and you should experiment a bit. Are those 3" fronts removable? I remember actually feeling the difference in vibration of the membrane when I moved them around but my hearing wasn't quite as discernable. With the traps, the bass went from nasty to downright evil, but in a good way.
For the price of the mondos, you could stack 2 Prime's (if you have 8' ceilings) and velcro them to the wall. You could lay them down on the floor if you don't have to worry about kids and clumsy vacuums. Don't worry, be happy.