Im a little confused on what "let her (demoness) wear her (violet) for a few days" means. I tried wearing violet and talking to demoness for a few days straight but nothing has happened so far.
Fight her while wearing her. (You might need to talk to her twice about that.)
Once you lose a fight while wearing Violet, she will take her from you.
Then you have to let her keep her for 3 days.
I reached this stage after signing the contract. Any way to give her Violet back? Or to progress even though I already have Violet back?
Wear violet and give up the battle. You might need to ask her twice for a battle.