The novel implies that Sonny and Lucy had something of a real relationship, rather than the two quickies that we saw in GF (she tells Dr. Jules, "Sonny and I did everything together"). And yes, I believe that if Sonny had lived, he'd have acknowledged Vincent and tried to do right by him.
Fredo seemed to be the repressed type. The novel describes him as something of a "Pappa's boy," still living at home in this thirties, which may explain why he acted like the kid in the nookie jar when he was spirited off to Vegas.

I don't think Michael would have made anyone an ideal husband. He was too controlling, too self-possessed, too utterly absorbed in building an empire, to have shared any real emotional closeness. The reason we never see him playing around probably was that he had no interest in women--only power.