1. I can only say that you need to participate in several duels while wearing this costume to obtain the skill (so that it's available in the Dark World).
2. As of version 0.75 Beta, you only need to gain access to these characters' castles. After that, their storylines end.
3. Basically, you need to help two characters from the Dark World, while ensuring that Celica is there without the annoying AI in the form of talking underwear and hats.