Bruce, your diagram is clear and raises a point which you illustrated but didn't discuss: when there are two rows of seats do you optimize the surround placement for the front row(where daddy and mommy sit?)or do you adopt a compromise location? In theory, when the setup is 7.1 the side surrounds should be directly to the side of the listening position to maximize the side effect, since being slightly farther back to provide a phantom back effect isn't necessary when there're actual rear speakers. Of course, with very wide dispersion speakers such as the QSs this is less critical.
My thought is to optimize the front row by moving the sides a foot or so farther forward. The back surrounds should be separated by as least as much as the distance they are behind the seats and wider is better than narrower. So I don't think that a deeper and narrower back wall location would be as good. The location you show for the back surrounds is good; possibly a foot farther back even better.
There's no 7.1 material, but using processing such as DPLIIx and Logic 7 to extract ambience contained in two and five channel material and sending it to the back where it belongs can be quite effective.
