In wall center channel

All I can suggest is try to line it up as close to the tweeters in your LR front speakers. That is what has always been recommended to me in order to keept he line of sound as it travles left to right as much in a the same plane as possible. Unfortunately not always practical when trying to keep the tv screen at optimum viewing height

When you install the center channel in-wall, you are introducing a number of diffraction effects into the system that are not tamed if the center was not designed to be in-wall mounted.

In my opinion, the best (home) theaters did not mix standing with in-wall speakers.

If you are interested in running a theater with your 2.0 setup, select some nice in-walls for hiding out of the way of your 2.0 setup - and, as Theo suggested - keep them lined up: the driver arrays often sound most engrossing when they are kept in the same plane.

There are many options; let us know your progress!