However, with all that being said, there is one particular detail that has troubled me. Now I am going off of memory as it's been a long time since I read Puzo's novel. Am I correct in remembering that in Puzo's novel the Irish gang shot Vito in the throat / chest area causing him to talk with a raspy voice?
In Puzo's novel he was hit in the chest. The raspy voice was introduced by Marlon Brando after he had watched tapes of Frank Costello during the Kefauver hearings.
Ok, I thought that the novel stated that he was shot in the chest.
So why write in this novel that he was shot in the arm? 
I believe that after reading the Puzo novel and reading that Vito was shot in the chest, Brando felt that Vito should now have a raspy voice.
And one other thing that troubled me was that no mention was made of Sonny seeing Vito kill Fannucci. In this novel he says that he saw Vito kill Tom's biological father, but never references his witnessing Vito killing Fannucci.
Once again, overall I enjoyed this book. But I am just a bit confused as to why these two key references from the original novel were not mentioned (Fannucci) and changed (Shot in the arm).
RichCerto?
