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?
Yes W, you make a point that I have made in previous posts. It's the film's timeline v. that of the novel and within the novel there are timeline related discrepancies.
However, I posted elsewhere what you did: why would the Senator refer to Michael's devising a murder plot , but completely ignore and never mention the murders themselves? It makes no sense: "Mr. Corleone, we know you arranged the murder of these guys, but what we want to know about is the devising of it. To tell you the truth, that was brilliant. Can you walk us through it?".
In additon, I find it, at the very least, incongruous that Vito and Michael would let nearly ten years go by before they carried out their revenge murder plot. Think about it: Michael and/or Vito could die; Michael and or Vito could come under singular law enforcement scrutiny; Michael and/or Vito could be imprisoned; Michael and or Vito could be murdered. Need I go on?
Consider
1) Vito had to train Michael how to be a Don, how the Family really operated, how to reach and maintain the mass of political contacts and other people on the Corleone payroll. Consider that it took years to train Santino for his position in peacetime.
2) Vito still may not have been 100% recovered when Michael came back--So he might've had to recover some.
3) Rocco had to build a secret regime, and he had to do it slowly so as to not draw any attention or suspicion from the other families.
4) The Family also began taking interest in the Casinos around this time.
5) The move to Nevada had to be planned and slowly executed, along with the sale of the Olive Oil Business. Imagine a GIANT Corporation moving the core of it's business--That doesn't just happen overnight.
6) Time had to pass to lull the other Families into a false sense of security and a false perception that Michael was a weak Don who could be messed with.
7) Michael (and thus the family) had to lie low for a while after his return given how controversial his return back was due to the murders.
8) In the interim, he got back together with Kay, married her, and had a son.
9) Business had to get back to normal after the Commission meeting. The War had ground the Family Business to a halt. Things had to start up again and go back to peacetime operating and regain lost revenue or territory (for example Sonny mentions blacks taking advantage of the Family's policy banks in Harlem) before anything major could be done.
10) Slowly seducing/recruiting informants in the other four families who could provide Michael and thus his buttonmen with where the other Dons would be to orchaestrate the hit properly.
11) The actual planning of the hit probably took time. A lot of things had to be considered: How to hit the other four family heads, how to minimize any potential blowback/retaliation, etc. How the hits would be organized and orchestrated so as not to restart a War.
Add in all these factors and you could see why it'd take Five Years. Consider that when we see Vito at the Comission Meeting, he doesn't look much different than he did at the wedding, despite being shot. He doesn't look much older. Now compare that to the scene where Vito and Michael are meeting with the Capos to discuss the Family's future--Michael and Vito are both at least several years older in appearance. Hell, compare how old Vito looks in his last scene to the wedding. His black/greying hair is now almost white; it's long and unkempt; He looks like an old man whereas at the beginning he looked like a middle aged, but still virile man.
