It's not enough for them to face one direction; another big factor that will influence the sound is the size of the speaker baffle i.e. the fact that the wall continuously surrounds the speaker. This impacts the dispersion. Free-air will sound different than in-wall, but because of the size of the plane that the drivers are on, not just because the direction they are facing or because the enclosure is in a wall.
I think you're worrying way too much about this. The M3's are wonderful speakers (as you already know). Unless there is something about the mounting mechanism that is unsuitable for horizontal hanging (instead of in a vertical wall), I don't see the problem.
Really, your requirements are (near) full-range speakers mounted in the ceiling. You probably have lots of options at your price point, but I doubt many of them are going to sound demonstrably "better" in that application that the M3 inwalls.
