As far as Christians not following the whole law: I'm not saying I agree with it, but Christians would explain it this way: That when Christ said, "I came not to abolish the law, but to fulfill it" (from your Matt 5 quote), that meant the fulfillment of the law in the Christian religion. Some practices of the law were no longer applicable (hence me saying above about super-sessionism and the reference to Acts 15).
I see what you're saying about the Christian critics of "barbaric" Islamic practices. Its unfounded and unintelligent, I agree. Their claim would be that such practices are no longer under divine law, since we are under grace and not under the law....you know, all that Pauline stuff from Galatians.
you say: "To put this bluntly: oftentimes Christians criticize Islam for certain things, and they are completely oblivious to the fact that we got it from you guys to begin with, since our religion affirms the revelations that came before us."
Ok, I see what you're saying, and I agree. As I said above, its really simplistic and unintelligent understanding of historical facts.
But to be clear, I find it difficult to identify myself as Christian anymore. I lost my faith years ago. But, I have PhD in Theology, so that's how I make my living. It just happens to be what I know.