"Actually, from what I've read, Laurents wrote the banter for Louise in the original production during out of town previews, so I'm surprised it's not in the licensed version."
No, he did not.
I am surprised to hear that you have received a script with the Christmas sequence and without Louise talking to the audience. I was also under the impression that the script currently licensed has all the banter. A friend has directed the show twice at a community theatre he runs, and he did the revised version of the sequence. I am pretty sure that's the version he received.