A Manic Pixie Dream Girl is a woman who opens the hero (who has been living a sheltered, emotionless existence) to adventure in his life. She's attractive, wacky, quirky, and yet obsessed with the stuffed-shirt hero. She leaves the hero opened up to life. In many cases she dies at the end of the story. Shadowlands is untypical in having the hero and the Manic Pixie Dream Girl be middle-aged, but that's not unknown (or even to have her be old, like Maude in Harold and Maude).
The classic form of this is Thorne Smith's TOPPER, both the novel and the movie. Pretty much all Thorne Smith's novels, in fact, start with middle-age men stuck in a rut who get their worlds turned upside down by the sudden intrusion of a wild woman and booze into his hitherto staid and stolid life.
As for it corresponding to Joy Gresham as she appeared in the film(s) but not in real life, it does correspond to her son Douglas Gresham's memory of her, and his conception of her effect on his stepfather.