Well, you're right, Vito--II has far more ambiguities than GF. Ending with Michael sitting on his deck with the gunshot reverberating, then the flashback, doesn't really bring closure. And I don't think either FFC or Paramount were envisioning III at that point.
I think SC is right--haste to complete the film. From time to time people here have unearthed many, many different scripts and scenes for II. I bet there are several different endings somewhere in Paramount's vaults.