Well, the first act is 1941. Little Edie is about to marry Joe Kennedy and Edith is throwing her this wedding celebration and is planning to give a little recital. Edie is worried about this, but she eventually kind of comes around to the idea. Edith's father is not and cuts off Edith's trust fund. Also, Edith recieves a telegram from her husband, Phelan, saying the marriage is over. Once some skeletons come out of Edie's closet, thanks to her mother, Joe doesn't go through with the marriage and Little Edie leaves Grey Gardens. Abandoned, even by Gould, Edith cuts herself off and says in Grey Gardens.
The second act, 1972 or 1975, not 100% sure, basically follows the documentary, but adds some more plot structure. All that really changes is that Edie almost leaves and decides not to.
I know I didn't include the songs in there, but does it answer most of your questions?
Some people paint, some people sew, I meddle.