From the storyline of GF2 which placed a strong emphasis on young Clemenza and from the correspondence between Castellano and FFC, I think it's very clear that one of the great shockers of GF2 would have been that Clemenza turned traitor after having been nearly killed by the Rosatos.
The party scenes with Pentangeli were obviously written for Clemenza. We remember him very well dancing the Tarantella in GF1. Thus - the ridiculous contrast in GF2 when the play "Pop Goes the Weasel". All those party scenes - the drinking scenes, the deleted "I show you a trick" scene, champaign cocktails, "He never trusted Roth" (Roth had been introduced by Clemenza) - were meant to show the alienation and frustration of Clemenza.

The Clemenza story in GF2 would have been: How can become the most loyal friend a traitor? It still works OK with Pentangeli, but there's no meaty background story about apart from some hints that he "worked with Michael's father".