The "buffer system," as described in the novel, was designed to protect Don Corleone against being convicted by a traitor in his own family. In order to obtain a conviction in a standard felony case, the prosecution must produce a "corroborating witness"--someone who can back up a main witness's testimony. So (a hypothesis) if Hagen had turned traitor and testified that the Don had given him the order to have the two punks who ruined Nazorine's daughter beaten to a pulp, the prosecution would have needed Clemenza to testify that he heard the order from Vito, not just from Hagen. If the order was passed down, one at a time, no witnesses, the Don was clear.
But you're making a broader analogy, UB. It applies in the case of Barzini keeping his role in the war hidden, and making it look like it was Tattaglia's and Sollozzo's doing. Yes, perhaps you could say it was a "buffer" example, broadly. But McCluskey was simply an operative, in effect an employee of Sollozzo's. He didn't care if Vito or anyone else knew his role because he thought he was, as a police captain, untouchable. The "buffer" analogy doesn't apply to GFIII. Yes, Lucchese was the ultimate foe. But, unlike Barzini, he showed himself early in the film ("Our ships must all sail in the same direction," leading Michael to characterize him and his pals as "a nest of vipers"). When Michael says "our true enemy has yet to show his hand" (just before his diabetic attack), he means Altobello, who was in fact subordinate to Lucchesi.