I know you bought a house back in 2012 I think and your HT room is 14 x 17? But I didn't know the ceiling was so low.
Regarding the M5 as a center, it depends on how far back you are going to sit from it. I tried it and at 14 feet back, I didn't like it. Don't get me wrong. As I've written many times, I love the M5 sound. It just doesn't work as a center for me at that distance. Your results of course may vary. Try one of your M22s or M3s. If one of those work, the M5 is a sure bet.
As for dual M5s as a center, I'd shy away from that.
