"They could have, among other options, promoted Robert Morse, who had played Barnaby in the film of 'The Matchmaker' up to Cornelius. "
That is a really intriguing idea that never entered my mind. Good for you.
There was some VERY weird casting of male leads in film musicals in the late '60s and early '70s. Jack Jones said in one of his club appearances that he was considered for Woody in the film of FINIAN'S RAINBOW. I think he would have been wonderful in the role, but instead, for whatever reason(s), they went with Don Francks, otherwise best known as the star of the Broadway mega-flop KELLY.