I am far from an expert, I only set up my first tank 6 months ago. However, I would suggest that if your tank is going to be placed in a corner, put the overflow box in the corner that will be against the walls for aesthetic reasons.
One issue that I'm starting to have with my tank is that the teeth on my overflow get algae growing between them after about a month. If I'm not diligent in cleaning it out, the water level in the tank gets about a 1/2 inch higher than normal, so that it's about level with the top of the teeth. You'll want to consider that when deciding how high to mount your overflow box. Depending on your flow, the water level will probably sit about half way up the teeth of your overflow box. You probably want the water level to be just at the bottom of your trim, so you don't see that ugly water line, but you still have some room for the level to rise if your overflow teeth plug up a little over time.
As for the return, you might consider just running a pipe over the top, or drilling it as close to the top as you safely can. You'll want the at least a portion of your return pipe to be above the bottom of the teeth in your overflow box so that you can put some anti-siphon holes in the line. These holes will also need to point, or at least angle, downwards or you'll have a fountain on your hands. If you don't get it shallow enough for anti-siphon holes, you'll need to install some kind of backflow prevention valve, which can be unreliable and need to be properly maintained.
Hope this helps, again, I'm far from an expert so maybe someone more knowlegeable will chime in.
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