The "modder(?)" who made Scarlett's expansion (it wasn't Bo Wei) said he will post a Walkthrough later, as well as a patch for the game making it a little easier to figure out. Parnaval is mostly right, though about how to progress though. Really wouldn't recommend using a Save Editor for it as you'll miss a lot of scenes/dialogue, which is sort of the point of the expansion (the actual "reward" at the end doesn't do anything besides new dialogue/flavor text as far as anyone can tell).
This is what is needed to reach the conclusion of Scarlet's expansion:
1)Beat her once.
2)Find her shoes.
3)Play the Shoe Challenge until she gives you the book of surrender.
4)Lose/surrender to her, unlocking the first training scene (I'm not sure it losing directly works, I just surrendered.)
5)Play the Challenge three times and pick each one of the rewards (the item one is necessary.)
6)Lose/Surrender to her, unlocking the second training scene.
7)Play the Challenge five times, and pick each one of the rewards (item necessary)
8)Lose/Surrender to her, unlocking the third training scene. Next time you speak with her, she'll further advance your training, unlocking the Serve option in the dialogues.
9)Play the Challenge five times, and pick each one of the rewards (item necessary), and play the three different Serve scenes.
10)Speaking with her again will unlock the option to become her puppet.
I did use a save editor, but I don't think I missed any scene. From what I saw in combat, the only thing that change is the foot trampling one, which is expanded to include the training stages when you lose/surrender to her.
I do hope that there'll be expansions for the other girls as well.
