Bill,
Your concern about twisting when horizontally mounted is valid, as I doubt the threaded insert is located along the center of mass in that orientation. My M22s are fairly old and did not come with a threaded insert. When I attached them to the Full Metal Brackets (vertically), I screwed directly through the brackets into the rear of the speaker with four round-head wood screws. If I were you, I'd forego the threaded insert entirely, locate the center of mass, and attach the mount at that location.
