as we know the Tattaglias and Sol saw through Luca's ruse. I have no clue how it would be the least bit plausible for Sonny to go to them and pretend he had differences with his father over the drug business. All this would have done was send a message to Barzini that the Corleones were not only weak, but that they were divided within the family. Talk about the Don Slippin....
Sonny would've been a long shot at best, I admit, but he would've been the only one at the time with enough clout to pull it off. Sollozzo/Tattaglia/Barzini might have taken it at face value and gone for it. Sollozzo knew that Sonny was at odds with his father about the drugs business, and he could possibly take Sonny's approach as a ray of hope for his deal. A long shot, yes, but better than the chance Luca Brasi had.
Remember though that there is no real division in the Corleone family in this scenario. If Sonny could pull it off and get Sollozzo to stick his neck out there, Vito still has Luca Brasi standing ready as his executioner.
