James, the 6' distance from the back wall is enough room to provide for a useful rear surround field. It's also enough to at least arguably allow a pair of small direct radiators such as the M2s to disperse sufficiently. If you like your QS8s now as side surrounds though, there doesn't seem to be a compelling reason not to also use them in back.
As far as positioning, yes they preferably would be about the same height as the side surrounds. A 2' separation wouldn't be enough to allow for both a left rear and right rear effect at the listening position. The positioning problem you mention isn't entirely clear, but a separation on the order of 6' or a little more would be desirable, if it could possibly be arranged.
