It's hard to defend a firm timeline due to inconsistencies within the canon.

For instance, Crazy Joe uses the Senate Chairman's statement "in the year 1950 you devised the murder of the heads of the so-called five families in New York" to support 1955. But how would he possibly know when the killings were "devised?" And why would he bring that up rather than the actual dates of the killings?

But, if we look past that and acccept the Chairman's statements as firm evidence of a timeline, we must consider also his statement placing the Sollozzo/McCluskey killings in 1947. With all the attendant publicity, it's hard to imagine that he would have mistaken that date by an entire year, which we would have to believe according to the proposed timeline above.

Also, I don't think it's likely that Buschetta killed Moe Greene. Michael's use of "buffas" indicates that he would not have given a kill order to a buttonman himself. That means that Moe's killer would have come from the ranks of Neri or Rocco - the very people Michael mistrusted. I think Buschetta was someone from the outside Michael had in reserve.