I've seen arguments on both sides when it comes to the NTR on this game and my opinion stands somewhere in the middle. I do believe Luna can be NTR'd, she's not inmune by any means. Your point about underestimating Iviel is spot on I think. She can handle powerful demons easily so the idea of a simple imp defeating her is inconceivable to her. The moment she accepted the contract she became vulnerable as we don't really know just how strong it really is and even if it starts out having little effect it grows in power over time, meaning Luna believes it's not a big deal at first and by the time she recognizes it as a threat it might be too late. The problem I think is with the pacing, towards the end the NTR becomes more and more unrealistic. Another reason for the complaints might be related to the choices and how they impact the game, from Luna's perspective it doesn't really make sense to give in to Iviel. They sould have given more compelling reasons to choose the "bad options" and the fact that the choices don't really matter in the long run is a dissapointment. My first playthrough I chose to give in every time until Iviel demanded sex, then I stopped because I felt like that was a line Luna wouldn't cross but it didn't change anything. The only real choices are all or nothing. Overall I really enjoyed the game as it fills a niche that no other game does or at least very few do, which is NTR games where the NTR is meant to be overcome. It's not avoidable NTR where all the content is reserved for the bad endings and the true ending is usually a virgin route with no scenes, the NTR does happen but you can fight against it and ultimately the game wants you to win and achieve the happy ending. I like the positive note that these games end on and I hope the developer keeps making the canonical ending the best possible one. Stories like Kurone's are seen as tragic and something that the main cast must avoid at all cost, hopefully even Kurone will end up having a chance at redemption and a happy ending.I dont see where you people get these opinions. Obviously you are free to have them but I completely disagree. Luna is perfect for NTR. She is loyal to Leo to a fault which makes her perfect for NTR. Her devotion is so strong that its her weakness. All you have to do is use that weakness against her and its corruption time baby. Luna practically looks like she is a Succubus Army character just with her design alone.
Leo is focused on trying to learn as much about the treasure as he can while trying to be defend the Mansion and find the traitor all at the same time and he doesnt focus on what is happening with Luna. And once Luna gets corrupted she becomes so desperate to save Leo and due to her blind faith in his character that she doesnt realize Kurone is starting to sink her claws in him. Since nothing else matters to her but saving him trivial things like doing something lewd (which she does on assassinations anyway) become a quick and easy way to reach that goal and not such a big deal to her in the grand scheme of things. She believes she could never lose to a low class devil and that ends up being her downfall. He blind faith and obsession with Leo caused her to underestimate her enemy.
And since the game is called Mirage's Defection it was most likely intended for her to betray Leo and the Human side. The Dev even said on Ci-en that he used to have a route where Luna becomes corrupted by Iviel and then Sadi steals her from Iviel. She was always meant to defect to the evil side. Corruption and NTR are the genre's the dev enjoys making so why wouldn't the main heroine be used for those things?