I have not read the entire thread, but I have thoughts. IMHO, co/tri-axial in walls are much better than 2-way MTM (Mid-Tweeter-Mid) or WTW (woofer-tweeter-woofer) arrangements due to how well they perform off-axis.
For a surround where you may be seated in a variety of locations having excellent off-axis response is crucial. Same is true for a Center. If you sit in a very narrow sweet spot this is less critical, and a Center can even be excluded altogether.
Also, really, I have a 5.1 and the flyovers are excellent due to extremely good speaker matching and room acoustics. IMHO, height speakers are oversold unless you have a yuuuge screen. :)
+1 on Monitor Audio by the way.

