1. I'm unclear on the difference between the M3 and M22. Does the application and room configuration above indicate one model would be preferable to another?
The M3 is designed with a slightly less flat frequency response than the M22 to simulate more bass and is often considered the better choice when used w/o a subwoofer. I have not heard the M3 so I can’t comment on that. The M22 sounds good w/o a subwoofer but for a lot of music I find it lacking without one.
2. What kind of power requirements should I be considering? The music will be played at ordinary listening levels, probably shaded to the quiet side in the evenings, and need not 'rattle the walls'.
If you go with the M3s or M22s and aren’t sitting to far away I imagine any midrange AVR or up will power them as loud as you might want.
3. What should I know about speaker placement re the wall of windows and the wall with the cabinet on it? If I put the speakers on either side of the cabinent, should I be concerned about the cabinet shielding the sound in any way...or should I be more concerned about the sound reflecting off the windows? I have 6' of space on either side of the cabinet, so I have plenty of room for optimim placement.
Generally you want the speakers place so they form an equalateral triangle with the center of the listening position. About 3 feet from any walls or other reflective surfaces is also a good starting point.
Curtains over the windows will help dampen any high frequency reflections if they bother you but the nice thing about curtains is you can also move them out of the way. Some reflections help create more 3D sound out of stereo speakers. With that big of a room you are going to have a lot of ambience. It also sounds like you will have a lot of flexibility in placement within the constraints you mentioned. The best, really the only, way to find ideal placement especially in a non-symmetrical room is to experiment with positioning.
You mentioned not wanting towers because of kids. I’m thinking that unless you shelf mount the bookshelf speakers they are even more likely to get knocked over if on stands than tower speakers are. Also the reality is that in a room that big you would be better served with full range tower speakers IMO. A bookshelf+sub combo is very effective and give more placement options than towers or even towers+sub but in a room that big I don’t think you’d have any issues with placing towers.
One of the best ways to find out what will work best is to order a couple different sets of speakers and try compare them in the same room. It’s the only real way to know what’s going to sound best. Return shipping costs are reasonable especially for bookshelf speakers.