if Michael did return that early, he'd have had to be Vito's apprentice for eight or nine years, given that Vito died in 1955 (film). That would be as unlikely as my original premise
I think it is not realistic that somebody, even though the son of a Mafia boss, rises to the top of boss in only a few years after becoming involved. I think eight or nine years being groomed for the top spot is perfectly normal.
