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.
I would echo John's comments here.
If that back wall is truly that far back, a direct radiating speaker will have less of a prominent localization than if they were much closer to the listening seats (try placing two speakers, side by side, 10 or more feet away, switch sound from right to left and see if you can really tell which is right and left...gets harder with increasing distances).
That being said, if you already have QS8s on the sides, sticking with the same for the rears is a safe and good option.
