Singularity issues.
Seriously though. I believe he mentioned he already pre-wired the side walls. But I agree with the above. Unless totally impossible, I'd use the front walls. I think front speakers that are pointed sideways to your seating position would definitely diffuse the sound more than you would like. After all, your fronts are your most important speakers, by far.
I would abandon the side wall jacks and find a way to wire to the front walls if at all possible. Carefully removing the baseboard is another place to hide them if wire invisibility is an issue. You often have to touch up the paint afterward but that's not a big deal if you are very careful. I assume there are also obstacles like door openings and such? Some imagination can probably get you around those too. Don't be afraid to run both speaker wires along the same wall. Ignore the guy at the big box store you may have overheard heard tell somebody that speaker wire runs need to be the same length. Electricity travels ant very near the speed of light. I'm very sure an extra 50' won't be noticeable.
As mentioned, rotating brackets or ceiling mounts are good technical options but I'm betting your significant other will hate that look. I know mine would.
Personally I'd rather have nice looking floor standers or M22s on stands than have in-walls if the only place to put in-walls was in the sidewalls, facing each other. If it's a WAF issue, feel free to blame me.
Just my thoughts, for whatever they are worth.
