If Sonny had not died and had made it to Carlo , would he have made good on his vow that if Carlo touched Connie again he would kill him?

I'm pretty sure he would have been stopped before actually killing him, but it's totally possible. Sonny was PISSED, and with his temper he could easily beat him to death or shoot him or something.

As the novel states, Sonny couldnever kill the father of an unborn child.

Well, this was the film. What was Sonny planning to do? Just pick Connie up and drive her to the Mall? Maybe beforehand have a chat with Carlo over a beer?

I cried when Santino died, most kids cried about Old Yeller. Me was Santino!

Well, those should be easy questions to answer. For one, Sonny tells Carlo "You touch -- my sister again, I'll kill ya." Then, after he gets off the phone after talking to Connie and finishes swearing, as he's hurrying to his car you can see the thought bubble above his head about what he intends to do to Carlo. I really love FFC's judicious use of those thought bubbles throughout the Trilogy.