AlanScott, THANK YOU for reproducing all of those quotes, with their sources. Very valuable to have them all in one place, for the record.
Just one clarification on the following:
***Douglas Watt, reviewing the soundtrack album in his Daily News “Record Review" column, made it clear that Nixon did the singing and that Hepburn was hardly heard at all. The review was titled “Marni & Rex." It’s not clear if he realized that Hepburn’s voice was heard singing the interlude in "Just You Wait." He does seem to have realized that Hepburn was heard at least in a few spoken lines. ***
I'm not sure why you used the word "interlude" here, as Audrey sings the beginning and ending sections of the song, and on the soundtrack album, Marni sings the entire center section, from "One day I'll be famous" right through "Guard, run and bring in the bloke."