The cocktail party before the nightclub wouldn't have taken any longer. Delysia wanted Gwenivere to help Edythe and her beau reconcile their differences, and Gwenivere did so. I think they made Edythe so dreadful because you don't meet Joe until the nightclub in the book. In the film, they built up a rapport over the course of the day, but in the book, they don't meet until 11 p.m. The Joe/Edythe feud made it possible for his eye to wander, and it also gave him the opportunity to come into the picture early in the film.

It's just a pity they thought making Edythe another villain was the best solution. She's really very nice and street-wise in the book.