I hadn't really considered an M2 for the center channel...let alone another M60.(Can you order just one??)
Yes, you can order just one. In the product page there is a link to 'add single speaker to cart' just below the yellow 'add to cart' and 'customize yours' buttons.
Why not order a vertical center and the horizontal center and try both to see what works best for you in your room?
There really is no way to screw this up, the 30 day trial period gives me confidence to experiment and make sure I get the right package for my needs.
I would also like your opinions on the "wall mount" version of the M2's , M3's and M22's.
I have the in/on wall M22's and in/on wall M2 for center (replaced my in/on wall VP150). Let me know if you have any specific questions. I like them very much but they absolutely need a sub. Once paired properly with a sub I don't feel like my system is giving up too much to the bigger versions. However getting the full bookshelf version will give you more flexibility in placement options, toe in etc... Compared to other on wall speakers I have heard they more than hold their own at far less of a cost.