I see you don't mention that Puzo states, just shortly after Sonny's death and before Michael returned home, that is was "1948". That's one of he few times a factual time reference is made in the novel. It was also made clear that Sonny was killed in late summer. And since Michael was on a two year exile, I don't think he could have returned earlier than 1948.

But since Puzo made so much time inconsistencies it's just too confusing to make a detailed and correct timeline. I think it's therefore bests to stick to the most basic information, based both on the novel and the films to make it more consistent.

This is what I prefer:

Last saturday of August 1945 - The story begins.
Late December 1945 - Assassination attempt on Don Vito.
January/February 1946 - Sollozzo gets killed & Five Families War begins & Michael flees to Sicily.
Summer 1948 - Sonny gets killed & Vito sets up the meeting with all the dons to make peace & makes arrangements for Michael to return home, which takes another year.
Autumn 1949 - Michael returns home.
Autumn 1950 - Michael reunites with Kay, but was already back for a year.
1954 - Michael becomes acting boss of the Corleone family & makes the trip to Las Vegas to have the meeting with Moe Greene.
July 29 1955 - Vito dies from a heart attack and Michael becomes the official Don.
Early August 1955 - Vito's funeral.
Late August 1955 - Michael arranges the death of Barzini, Tattaglia, Stracci, Cuneo and Moe Greene & makes the move to Nevada.