Sorry to say, u r wrong. The challenger toppers has to prepare for two songs, because they get 6 days between their performances. The same thing happened before and can happen again. This ihas nothing to do with any set up.same reason why the lowest scorer from the champions side gets an exemption in the same week. Because he/she only have one day to prepare.
You may be right! I was just speculating since Aneek's song was very tough and needed a lot of preparation. Or maybe, they plan out their themes a few weeks in advance and contestants are asked to prepare for them a couple of weeks ahead of time.