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.
1) They might've gotten from Frankie or Willi's statements that Michael had planned the murders of the heads of the Five Families as early as 1950. It seems like Frankie gave the Senate Committee EVERYTHING he could give when he made his affidavit. Which makes sense if you consider the deleted scene in GF I with Michael and Vito in the Garden where Michael tells Vito that he gave his word he wouldn't break peace, but Michael didn't, that Vito would have no part of it--The beginning of the vengeance. Add to that a few years spent in Michael trained how to be a Don, along with preparations to move the huge Family business to Nevada, and more time in to have Rocco build up a secret regime and you could see why it'd take from 1950-1955. Also, the Massacre takes place after Michael and Kay's talk (about the Corleone family going legitimate), which was in the fall of 1950.
2) Remember that whether it was 1946 or 1947, it was still more than a decade before the events of GFII. Yes there was a lot of publicity about it, but it was over a decade ago, old news. It is possible the Chairman made a simple mistake.
