In the interest of keeping this merry-go-round going:

The novel states that after the Dons' meeting "But it was to be nearly another year before Don Corleone could arrange for his son Michael to be smuggled back ..." Even if the Dons' meeting tooking place in the summer of '46, Michael would have been back in the States by mid-47. No where does it state '48.

In the film, Michael tells Kay he's been back "A year. Longer than that I think." So, (as the novel states) if Kay had been living like a spinster for almost 2 years, then Michael came back in '47. Thus he had plenty of time (3 years as the novel states) to learn the family business prior to the Dons' massacre in '50. Stiil, the novel states that Michael's exile had deprived Michael of 2 years of valauble training. The beat goes on.