You wrote, "So maybe Sondheim is just quick to anger when it comes to 'Porgy and Bess' and he would have attacked anyone who proposed making those changes."
I've been staying out of this, but since you wrote that, I will jump in just to say that the love and reverence he feels for Porgy and Bess is surely part of it. I've seen him start to cry when he simply talks about Porgy and Bess. He does not feel that way about Oklahoma! When asked to make a list of musicals that he especially likes, Oklahoma! was not on the list. Very little was on the list. The Rodgers and Hammerstein musical that he loves is Carousel.
EDIT: I now see that larry13 made the same point before I did.