Have you also explored the possibility of having your screen mounted diagonally across the top right corner, and using a right angle sectional facing it, having the focal point of the "V" opening/facing the top right corner leaving space at each leg end of the sectional as a walk around? as Dean mentioned, for optimal placement of the surrounds, you might consider ceiling mounting if side/back placing proves unacceptable.
I've recently bought some top notch sofas and chairs at Sears Outlet for roughly 30 cents on the dollar. They had quite a few sectionals on the floor, at all the outlet stores I went in, you just have to keep your eyes peeled for that "one day 40-50% off already reduced sales" that come up quite frequently(here).
