What a great post, fastbolt! Most thorough commentary on the "guts" of a M&P that I've read. Though lacking your knowledge, I was also impressed with the inner workings of the M&P (though I thought the little bit of sponge in the trigger spring (as I remember) was kind of weird).
I had a M&P 9mm full size that I really liked. I bought it for shooting plates. It never missed a beat over 3,000 rounds of cast bullet reloads and it was very accurate. My best group was .383" with a Lee 105 gr SWC and Win 231 offhand at 7 yards.
I agree that, with the small backstrap, the feel was very close to a 1911. I did the online trigger job as posted on the M&P forum (I approached this very conservatively) and it turned out very well. The magazines were very well made though I did have a couple of misfeeds which I think were due to not cleaning them. (I used cast bullets lubed with Lee Liquid Alox which is kind of dirty.)
I sold the M&P very reluctantly as I really like everything about the pistol. The problem? I just couldn't "run the plates" worth a hoot with the pistol and I never could figure out why. I did much better with the 1911 platform and even with my CZ75 Compact. Maybe I was just destined to shoot steel pistols with hammers