Now tell me something. What excitement would there have been if we actually saw Carlo make a deal with Barzini's people or call them up after beating his wife?

I believe that FFC's whole premise was to make the viewer feel the uncertainty about Carlo setting up Sonny just as Michael felt that uncertainty. While we know deep down as well as Michael knows deep down that Carlo set up Sonny, there was still a slight window of doubt because no one could coroborate that Carlo was guilty. That is why Michael had to make him confess. Watch Michael's face once Carlo admits that he set up his brother. A combination of disgust, sickness and an I knew it all along look comes over Michael. Up until that point we can see that Michael is still a bit unsure himself. Then the confession, and that cold classic Michael look appears.