Michael, that should work well. A few inches of "breathing room" for the rear port is adequate. In general it's all right to have a speaker very close to one room surface as long as the distance of the woofer cones to the other two room surfaces(e.g. floor and side wall) differ from that and from each other. One point would be not to set the M22s at all back on the shelf but to instead have it slightly overlapping(maybe 1/2")the front edge, so as to minimize diffraction caused by the shelf.

-----------------------------------

Enjoy the music, not the equipment.