I made a case for Michael's early return based on quotes from the novel, putting his return either in late '46 or early '47, in several scenarios. But then you pointed out that, 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: that the war couldn't have lasted nearly five years.