My thinking exactly. I believe he should not consider himself better writer than Puzo. Everything he wrote as sequel to the original leaves me confused at least. By the way, Kay became a strict Catholic... And I really don't see reasons to leave him NOW. If she would leave him, it should be when she did it first, and if she returned then, after all, she did it knowing everything, and accepting him as he was, with all that Sicilian things. It would be strange to despise him now for shooting in his home - as if he wouldn't prevent it if he could! If she loved and respected him as she did in the novel, even knowing what he was, she would never change so quickly just because he had difficult times now. It would be still less normal for Kay not to show him any kind of support, understanding and compassion at all. She knew perfectly well whom she was marrying, she was clever enough to see that he never will be absolutely legit, and if she decided to stay with him despite everything that happened, it is absolutely out of character to change her mind suddenly and begin to kill your children just because someone shoots at your husband (great surprise, indeed!) and you are requested to stay at home for the safety of your own children (as if any sane mother in her place wouldn't put off all her trips without being requested!). She could not be so light-headed.
It's just another violation of logic performed by FFC. He seemed not to understand that you cannot just take existing characters and change them the way you please, without taking anything into consideration. Michael's character suffered from him even more.