Solved

Questions about Off-season

I am a league administrator. If I change the number of off-season days during a season, will it be reflected the day after the current season ends? Or will it be reflected the day after the new season ends?

Im not sure how you would do this except to postpone the first race of the season just after the new calendar is released and results in the one day between seasons. You can choose that way how many days

The season break setting is only visible in the desktop version of the game. 
This setting should be active after the season when you change it during the season.