Rich,
Keep the tweeters towards the inside, not the outside, for more coherent imaging. I think I'd keep the FMBs vertical to keep the stress on the bracket even (this is just speculation on my part; I'm fiddling around with the FMB as I write this). It may not matter. Perhaps someone who might have mounted the bracket horizontally will respond.
The M22ti's do sound somewhat different in a horizontal configuration but they still retain their tonal balance, neutrality and transparency. Before you drill holes in the wall, you might try them out in a vertical and horizontal orientation while you listen to them. I doubt you'll hear much difference, but every room contributes its own sonic "footprint" (combo of direct and reflected sound) and there may be differences in your room.
Overall, in my experience, the M22ti is especially adept at delivering excellent sound in even "difficult" rooms and despite far-from-optimal placement.
Regards,
