For some rerason I don't think Fredo would have betrayed Sonny they way he did Michael. If anything, Sonny probabloy would have been smart enough to give Fredo "something" even if it wasn't really "something." Fredo feared Sonny, but he underestimated his younger brother.
Of course the whole premise of Sonny being alive turns on whether he dodged the assassination attempt, and it implies that he ended up deciding not to go into the City to kill Carlo. This leaves the question what would he have done with/to Carlo?
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
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