I have trouble picturing Scarlett Johansson as being "corny as Kansas in August." Nellie is a bit of a goofball (see the scenes of Mary Martin in the original production). I think that's FAR more important than how young she is. The worst they could do would be to cast some overly-serious dramatic actress in the part who weighs the entire production down like a lead balloon (see the TV movie with Glenn Close).
I think Reese would be a great choice for Nellie.
I also think it's very important that Emile be a singer. A seriously fantastic singer, not a shower singer movie star. They could turn to opera and use Paolo Szot, or they could cast some internationally-known singing star.
But if you start calling on the ranks of celebrities who can "carry a tune," the whole thing will start to smell. I don't need to hear a breathy-vibratoed, pitch-stabilized "Some Enchanted Evening."
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22