Without confirming with Ian, I would have to say that the W series are designed to be mounted on wall. I recommend not to attempt flush mounting them as you will increase the boundary effect and produce a hump in the lower frequncies, thus "colouring" the sound.
If you absolutely must have inwall, I would also suggest looking at Monitor Audio CP series. They (as doother flush mounted inwall speakers) have switches that allow you to customize the crossover settings to reduce the hump created by the boundary effect depending on the proximity to adjacent walls (or have no compensation).
As for surrounds, you CAN mix and match unless you have a desire for a matching set of speakers. Heck, I have a friend who would not buy a receiver unless it had blue LEDs.
If you are thinking of inwalls for surrounds I would not recommend it unless, again you have a desire of everything blending in with the walls. Companies like Triad build nice inwall dipole spakers but I think you lose some performace as they tend to become a little more directional (which is not really what you want in a surround effects speaker). However, I believe they do this primarily for aesthetic reasons and I am sure it does not "sound bad".
Perhaps you should look at mounting your acoustically transparent screen about five inches from the back wall (ceiling mounted?). That way you can mount the W series with no interference with the screen.
I sense that you are trying to achieve the best possible sound quality
and have them "invisible". Unfortunately you can not really have both. Inwalls will never quite be as good as properly positioned floorstanders or bookshelves. The W series are IMO a step up from most inwalls due to their design inherantly reducing the boundary effect that colours the sound. There is only so much one can do to compensate for acoustics. That being said, you can still achieve a decent sound with inwalls.
