In the gallery menu, there are a few individual girl and random event scenes that are not unlocked, but the hint section states 'that's all for now'. Does it mean the scenes are only available for premium version, or are they unlockable but there's no hint for them?

All scenes in the gallery should be available. If you follow the achievements you should be able to go thru all the content in the game.
(achievements can be found in the INFO app in the Phone menu)

I don't know how to get the one on page 1 number 6