BUT: since Pentangeli replaced Clemenza in GF2 because Castellano withdrew, it makes sense to imagine Clemenza in the same situations. In GF it was Clemenza who danced the Tarantella enthusiastically and drank wine like an elephant drinks water. Neither wine nor Tarantellas he could enjoy at Anthony's communion party.
And a lot of the tragedy-impact got lost. Because with GF and GF 2 we would have seen all stages of Clemenza's development from a young criminal until his suicide. The court room scene included a noteable parallel to Castellanos real life who had family relations to the Mafia: When the wedding scene in GF was shot, a man appeared at the set. He talked shortly to Castellano, and it turned out later that this happened to be his uncle - Paul Castellano who told him that the family approved of his appearance in the movie.
Clemenza would have been the one to be strangled by the Rosatos, just as he did to so many others, most notably Carlo.
A misbehaving Pentangeli at the party is just a funny guy. A misbehaving Clemenza would have been a tragic character, especially because he had so much self control in GF.
As far as I can tell from Ardell Castellano's book she talked him into withdrawing. Coppolla had written two very friendly, personal letters which she interpretes crudely. To me it seems to be obvious that she wanted the job as an editor of the script who has the final say about the lines, which of course FFC could not accept.