Yes: eating spaghetti all night is definitely Turnbull-like!
I agree with SC's analysis about the strings. And I agree with your point about the Corleones' unhappiness. In fact, for all the power they had, Vito was powerless to prevent his daughter from marrying a guy who he knew was a bum; his son from being murdered by the bum; and his most favored son from being sucked into the family business. The fact that most of this happened while Vito was incapacitated is a nice metaphor for powerlessness that might even call into question the string-pulling analogy.
Michael's even more pathetic because he's even more frustrated: Constantly seeking "legitimacy" and constantly being exposed for the criminal he is. How much power and happiness does a guy have when, near the end of his life, he wails, "Just when I thought I was out, they pull me back in"? As I've said many times, Michael became the biggest criminal in the world, but he spent his life winning battles and losing wars. That's why I think the three-word summary of the Trilogy is: Crime Doesn't Pay.