Your description is very similar to how my room is setup currently. My rear seating row is up against the back wall with the rear surrounds mounted as you mention. They are apart about the width of the couch. The side surrounds are mounted a little higher and angled down toward the listening room using the Axiom mounts. Don't worry about not having one wall open, at one point before I enclosed my theater will walls, I had 2 sides wide open.
Anthem AVM60 Outlaw 7700 Emotiva A500 Epson 5040UB