Without any doubt the biggest contradiction in the entire timeline is the 55 massacre being said to take place in 51

This makes no sense because:

1. Vitos tombstone says 55
2. if we go by the timeline said in the second movie of the Turk killing in 47 then its saying that Michael left for Sicily, came back, married Kay, and had a 3 year old all in less then 4 years. This is pretty far fetched because we know he spent at least a year in Sicily and didn't see Kay for at least a year coming back to america