symmetry of room for mono?
Mono room more forgiving. Most program material is all-mono or mostly-mono, even though delivered on formats with potential for wide channel differences.
2 channel stereo never provides "solid" center (for more than the theoretical listener in the "sweet spot"), 3-channel "stereo" is not distribution standard, but provides far superior listening for a small group.
I think that you are right to place loudspeaker on long wall to give side reflections more time and distance to diminish.