I always considered the hit on Sollozo and McCluskey as the turning point in Michael's life.
However, reading all these thoughts, got me thinking or maybe speculating is a better word. Perhaps Michael knew he was "in" the business after these murders, and yet perhaps had Appollonia lived he may have tried to go back to normal.
I think of that short scene when Michael was walking in Sicily and Don Thomissino drives up to him and Michael asked "Did they say when I can come back to America?". Then too, the loving looks and gentle manner he had with Appollonia during the dinner at her house. I don't know these seemed more down to earth and lacking of any of Michael's later "cold" traits.
Then again, if Puzo would have wanted the story to go that way, he would have written it that way. Still it's interesting to speculate since we all seem to know these characters so well.
TIS