After painfully wading thru this, I think I can sum it up for those of you who start at the end:
Anyone who wants more megapixels than "me" is an idiot and doesn't understand picture quality.
Anyone who wants fewer MP than "me" doesn't do real photography.
Ansel Adams still rocks.
I didn't wade through all of it.
I'm skeptical of an 18 mpix sensor for 5D3. Why would Canon put its new flagship 1Dx sensor in a lower class body right away? Doesn't make sense to me. I'm still thinking 36 Mpix. Canon will respond to Nikon in this regard.
FWIW, the diffraction thing. 36 mpix is the same pixel density as 14 mpix on crop. So from my experience with 18 mpix and 10 mpix crop sensors: you're good at f/6.3, okay at f/8. At f/11 you start to see degradation and f/16 is for smaller prints only.
People have fretted about the diffraction thing every time a new crop sensor has come out. IMHO 36 FF (and 14 mpix crop) is just about optimal, and beyond that, you start to get into diminishing returns.