can bipolar surround speaker on backwall with 5.1

If you haven't already purchased the receiver and speakers I highly recommend a 7.1 HDMI receiver and seven matching Sync One B's or smaller speakers and a sub. If your couch backs up to the back wall the rears would mount in the upper rear corners on the side wall pointing down at the listening position. Mount the the sides at ear level next to the couch. The L and R at ear level and on the same plane with no toe in as close to the display as possible with the C either over or under the display on plane with the L and R. With the calibration mic at the listening position let room correction do the rest.

No room is too small for 7.1 and matrixed 7.1 sounds better than 5.1.

Since I didn't read your well written room description closely my suggestion of surround and rear speaker placement is not correct.

My HT room is similar to yours except my 60" display is in a corner which would seem even more awkward for a good surround presentation but the Pioneer room correction is amazing. My description of the L,C, and R, are as I described above. My surrounds and rears are mounted very near the celling aimed at a 3' wide sweet spot. each of the surrounds and rears are at different distances from the listening position.

Previously I had a 5.1 Integra system which seemed to work well but upgrading to 7.1 with room correction has created an outstanding sense of surround which comes mostly from 5.1 media. Replacing the towers with matching speakers and adding a hidden sub will be much more fun with much deeper and controllable bass while giving the system a more sensible look.