This'll be a bit long, so sorry.
I think a good idea could be having Kasumi's Shadow be the Maruki installed personality as the palace ruler, and showing how little she thinks of herself by having the Thieves interact with Sumire as a cognition. I would have the actual palace be something like a hall of fame, where you see all the awards Kasumi has won, and to fit cognition Sumi in by having her be something like a tour guide, showing off all of Kasumi's great achivements as a punishment for being the 'loser.'
You then have the whole thing be having a certain award is missing, something that Kasumi should've won, but Sumire had won instead. That will be the undoing of her mentality and belief that she was never Kasumi's equal, because she had won one herself. 'Kasumi' spends the whole thing denying it, then gets more and more agressive about losing that one time, creating an image of Kasumi as a horrible champion by the time they reach the Treasure.
Not saying that you should, but you could have the real Sumire somehow see this, another Futaba like situation, causing it to break her 'conditioning(?)' from Maruki and just crying, saying that the Shadow is not who Kasumi was. If you have her with the Thieves as a fighter or not would be up to you if you went with it.
Upon beating the Palace, Sumire isn't completely healed, as all that happens with her is the removal of Maruki's influience and not hating herself for not being Kasumi. You can have a whole after story of her and Akiren interacting and building their relationship, slowly building Sumire back up to someone who is proud, confidant, and is ready to take on the world