Since I'm a Maggie fanatic, maybe I can be of some help, or at least muddy the water for you a bit.
I currently have Maggie 3.6Rs and a CC3 center in the front and 1.6QRs as surrounds. I also have a pair of MC1s that I use in my bedroom. I've been waiting patiently for over a year for Magnepan to release their new true ribbon center speaker, at which point my CC3 will get back speaker duty.
I strongly recommend that you spend the extra money and get the CC3 center. It will be a much better match sonically for either the MC1s or the MG12s. Plus, it will give you some room for upgrading should you decide to go with larger Maggies like the 1.6QRs at some point.
If I had a dedicated room the size of yours, I would opt for the MG12s with a CC3 as the center. I would probably forego the MMG-Ws altogether and use the MG12s as surrounds also, since I listen to multichannel music quite a bit. The MC1s sound very good, and wall mounting is certainly a plus, but about the only way to find out which speaker is going to suit you best is to audition them at a Maggie dealer or a Maggie owners home. No speaker should ever be purchased blindly, but this is particularly true for Maggies.
My only concern with your setup is your receiver and it's ability to handle a 4 Ohm load. I'm not familiar with the Onkyo 805 and it may be fine powering the speakers at normal listening levels, but you may have issues if you like things loud. You may want to contact Onkyo customer service and ask them how well the 805 will handle 4 Ohm loads.