I don't think there's anything wrong with using M22s as rear surrounds. Whatfurrer runs the 22s as his side surrounds and is very happy with them.
I think the reason the M2s are mentioned more often as rear surrounds is because there really isn't that much rear surround information that requires more than the M2 can deliver.
With that ratio of HT to music, you might consider going with QS8s as the rear surrounds. One reason direct radiating speakers, like the M2s and M22s, are often mentioned as rear surrounds, is that direct radiating speakers will serve you better when listening to music in 5.1, particularly when listening to SACD and DVD-A disks. If your stepfather is not interested in that, then the 4 QS8s in the rear would give him a really immersing HT experience.
I recommend you run this question by one of the folks at Axiom. They'd be more than happy to advise you on what to do, and why. It would be well worth the cost of the phone call.
