Introducing Molly's lovely orange sprigged dress! It is simple and tasteful, quite "Molly-ish". She wears it quite often throughout the movie, particularly in Episode 1. It is the first dress we see her in as a young woman.
This is the best picture I could get of the back closure. It seems to overlap, and probably closes with hooks-and-eyes.
You can make this dress by using (and altering, a little), Sense and Sensibility's Romantic Era Dress. I recommend using the small bateau neckline option and trimming it with a wide bias (study pictures for the details of the neckline). You may want to raise the bottom of the bodice a couple inches, and gather it. Use the back closure option. Molly's sleeves here are more narrow that the sleeves offered in the pattern so you may want to alter those so that they are not so full.