Given the long history of Clemenza and the Corleones, and the differences between Clemenza's and Frankie's personalities, I don't think that simply substituting Clemenza into the Pentangeli plotline would have made for a plausible betrayal.
I think Clemenza's presence in the film would have required a very different tack to make the betrayal plausible and tragic.
You hit the nail on the head.
I think it's a question of acting. Just by reading the script, Pentangeli's betrayal isn't very plausible either.
If we had Castellano in GF2, some of the scenes would have been more hurtful to
Clemenza. Imagine these scenes with Clemenza.
- The Tarantella/No Tarantella scene. Compared to the wedding where Clemenza was a guy full of joy he was humiliated, everybody was laughing when he left the stage.
- He had to wait in line, which really was a humiliation, not a sign of old friendship.
- He had to drink water from the hose instead of being served.
- In the "Champaign cocktail" scene, Clemenza must have felt that he had become a pawn in Michael's game.
- He was about to be garroted just like what he did to Carlo.
- The cruelty of bringing Clemenza's brother to the court.
