I'm afraid I can't contribtute much to your conceptual questions, although I've often wondered the same things myself and agree that most in-walls are lacking in one respect or another for 2 channel listening. The sound stage is usually compressed for one.
I'll second Albert's comments concerning the B&W Sig 7's. I'm quite happy with a pair that are being used as the rear surrounds in a 7.1 HT (they are mounted in a tray ceiling in my kitchen which opens into the listening area). I've also been planning to use another pair for 2 channel listening in a bedroom system. To experiment, I placed them on top of a pair of Talon Peregrines that were sitting on Sound Anchor stands (to get the right height). I was pleasantly surprised at the results and will be installing them shortly - I can't say that I preferred them to the Peregrines but they were pretty close and it solves the space problem nicely.
I belive the higher end Niles in-walls allow for all sorts of adjustments to fine tune the off axis radiation pattern, which I suspect would make a big difference in terms of sound stage. It's equivalent to achieving the optimal toe-in for floorstanders.