When I watch those thirties comedies, especially the screwball ones like Midnight,Awful Truth, or Easy Living...I can't help but try to picture them in musical comedy form. The format and pacing of these films lends itself easily to the musical comedy format.
I love the suggestion of York in Lombard's role. She's always impressed me as having good comic timing, and her voice is great as well. I, too, am hard pressed to think of a suitable actor to fill the Godfrey role. There are no names that come to mind immediately. Out of all the names you brought up, lovestheclassics, Brian Stokes Mitchell comes closest to fulfilling the vision of Godfrey for me.
I'd have so much fun with the Cornelia role casting, though. As written, it's pretty one dimensional, but what fun could an actress have with those lines! After watching Anastasia Barzee recently in White Christmas, I'd like to suggest her. She played ice queen well.
I'm glad to see that I'm not alone in my enthusiam for projects such as this!
"My dreams, watching me said, one to the other...this life has let us down."