You're overflow end looks really good.
The far end pushing returns... well, personally... speaking from experience... I would copy my design
If you don't do the mixing eductors (which I think you should) at least don't bother to connect the returns.
As you already became aware... the plumbing jams nice and tight in those corners.
It pretty much holds itself in place!
I would not bother to drill any trim...just zip tie directly to cross braces.
Algae will "paint" the returns quickly so I strongly feel painting it black would be a waste of time.
The return plumbing on my aquarium isn't noticeable except for the eductors (which happen to be black)