A lot of character special information is missing from the english version. I believe the details of the magical contract Luna forms with Ivieru are supposed to be in the game dictionary. Then again, even if it was visible, it probably wouldn't give more info than what you initially get at the end of chapter 2.
From what we see with Kotori, the contract directly talks to the victim's subconscious to slowly make them look more favorably toward the contractor. That is the premise for Luna's corruption, to resists it before the final day. Although Kotori instantly falls, there was nothing gradual about it, but maybe the Dev was illustrating that Luna has a stronger will, so she wouldn't fall as easily. But based on game events, it isn't only the magic that corrupts Luna, but also Ivieru creating favorable circumstances to get Luna to let her gaurd down,
If you go the NTR route, the narrator explicitly states:
Ok, now this make me clear on how to see this game.
1. Luna NTR route
All luna ntr scene is showing to us if luna is easly get panic and ivieru can use that rift, like she told to leo before facing ivieru for the last time. She can easly get swift by how "usefull" ivieru is and slowly she get ntred by him.
2. No ntr route
Its show us how strong luna's mind and will. It also show us how weak Ivieru to do provocation and how poor his plan.
What missing from this story
1. Consequences (read it whit john wick voice)
There is no consequences from what luna choose. When she disobey ivieru, she doesn't get punishment or something that make her regret of her disobedient and ivieru still make the dispell potion for her. Leo and Kurone relationship also doesn't have impact to ivieru and Luna relationship, even though it can be used by Ivieru to seduced Luna.
2. Progress
You don't need a progress, don't care how corrupted Luna you still can save her, don't care how pure Luna you still can ntred her. You can fuck Kurone how much you want and you can still have true route. I know there is something change when you doing ntr route, but its not enough. Luna and Ivieru should do it more and more, increase the intensive, show us that ivieru make a progress.
I understand that the game event is only 3 weaks, i can tolerate it if its need to rush. But non ntr route sounds more make sense because there is no consequences and solid progression, only good things goes smoothly, even ntr route can cancelled by only force kiss.
I know i already made my point before, but yeah