who was the actress in bed with Charles MacArthur

You mean when she shows up unannounced and finds the half-naked woman in his bed? That's Gwyneth Paltrow's character...if you remember, later on D.P. steals away the psychiatrist at a party while he is talking to Paltrow's character (sorry, blanking on her name & too lazy to look it up) and says to her, "Do you mind?...Well, at least I asked first." Hope it helps!

Ive often wondered if her character "Paula Hunt" was based on the real life Claire Luce Booth with whom Parker had many run ins with, resulting in some of her best jabs.

Once when leaving a hotel Booth stopped and held the door open for Parker and said, "Age before beauty my dear". To which Parker replied while stepping through, "Yes, and pearls before swine!"

MacArthur was a womanizer. He was the father of the child that Dorothy Parker aborted, which is referenced in the documentary, The Ten Year Lunch. Helen Hayes was very Catholic, so MacArthur cheated on her frequently while Hayes, evidently, looked the other way. I remember that they had a handicapped child of some sort and she focused her energies on the child and on her acting career.
I expect she breathed a sigh of relief when MacArthur died and left her a widow.