Not sure if anyone has mentioned it before, but I've just found a way into an in-game 'development room', guarded by the ultimate-fuckery of a boss called 'Space-Time Mouse' [I actually has to cheat things to defeat it]; the room is located in-between access to 'Abandoned Building' with the 'Casino' in the 'Slums' [behind the 2 NPCs with a fire-place]...
In it, was a legit function to unlock all Scenes and their CGs along with 3 additional development notes; however, I currently don't know how to get it unlockable [as I kept seeing it over-and-over, but it still says 'Locked', until I tried it again near end-game] - I'm so desperate about finding clues to get CG-56 and CG-60, but no one has the answer... so finding this in-game gimmick sure was worth it.
I did, however, completed the following quests... not sure if they were related at all though:
- Akihito's Memo
- Youma Core
- Dojo Martial-Artist
- Shrine's Ghost
[My guess would be the 'Proof of Martial Mastery' item though - it is the last 'unknown' item I've gotten in my first 800-minutes of playthrough]