I think he was just trying to write the book as a sequel to The Godfather without contradicting the movies. He just depended too much on the movies for him to get inspiration. But yet, he still does things that do not go in coordination with the book. Like Vito's mother's death. And also, Francesca married off to some other WASP, not a billy van asdale. You can only see this though in a deleted scene for GFII.
"I never wanted this for you. I work my whole life--I don't apologize--to take care of my family, and I refused to be a fool, dancing on the string held by all those bigshots. I don't apologize--that's my life--but I thought that, that when it was your time, that you would be the one to hold the string. Senator Corleone; Governor Corleone."
-Don Vito Corleone to his son, Michael
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