I would probably stick with 5.1 for the time being unless you want to mount speakers in the middle of your ceiling. Regarding placement, you want your side surrounds above and slightly behind the listening position. I would try and move them forward some to maybe a foot behind the couch, then aim them down at the couch. Those clamp mounts should work, they seem to be pretty popular. I would recommend a second sub over the extra surrounds, you have a big open space so a second sub will add some output, but more importantly improve the quality of the bass.
Denon AVR-1713 | EMP E55Ti | EMP E56Ci | EMP E5Bi | BIC F-12