Caught up on the episode today and this part jumped out at me too. Even though he referenced it a few times, I don't think Simmons read the novel. If he did, he'd know what Michael was up do after the war and before the wedding...he was in college at Dartmouth with Kay. It is referenced a couple different times in the movie. Sonny's line about "blowing his brains all over your nice Ivy League suit" being the most obvious.
Yes. Again, if he'd read the novel, he'd have a better grasp of Fontane's back story. He was aging out of his heartthrob crooner days and losing his singing voice, so he wanted the film role because it would give him a new career where he didn't have to rely on his voice. I think Simmons was too married to the idea that the role was based on Sinatra...the back story yes, the whole character...not so much.
Obviously since those details aren't in the movie, it's hard to fault a guy for not knowing them. But for as big a Godfather fan as Simmons claims to be, you'd think he'd have read the book at some point.
I read the book(probably over 15 years ago), I forgot those details.