You do know 'conditioning' is from a narrative perspective its what we call character development, do you think you are who you are nowadays without having been conditioned heavily to become it? You are conditioned by your parents to think some way, conditioned by your peers to think some way, conditioned by your education to think some way, conditioned by your entertainment to think some way, conditioned by the media to think some way, conditioned by the government to think some way, etc... and that include being conditioned by your partner to behave in a way which is more appreciable for them, if you think otherwise you are naive, you are fundamentally conditioned against your will by so many different forces in the world to be what you are now, and if you think all those forces do it in support of your current leanings, nope, if you think they do it for your benefice, nope.
The point is you see any changes to the character personality induced by anyone else as 'conditioning' well yea but if you think its abnormal you have lived in a cave for a very long time. Its how society has and will always work at anywhere from the micro level to macro level. Its life as a human being. Sure every now and then a change is born out of self reflection, but that is a minority of the changes someone undergoes in their life, the majority are induced by external forces.
You can complain about this, but what are you doing in real life? Have you taken to activism to change the world? Or do you just either accept or escape reality in fiction? Their is a reason the world is shit, because 99% of the population have been conditioned to ignore the fundamental issues that afflict society, so they just stand by as everything goes to Hell. If you expect fictional stories to be any different you ignore the fact that all fictions are based to some degree in reality.
Anyhow if at the end the MC agree of his own free will to be in a NTS relationship with Lacey, because thru this story his mindset on the subject has been changed, well it is what it is, its character growth, just not in the direction you like it. And before you think I am a fan of NTS, I am not, I am not arguing all of this because the idea of the MC & Lacey being in a NTS relationship makes me wet, it doesn't, I am for effectively in for a harem ending, aka 'big family', you can say it doesn't match the MC & Lacey's personality, but it doesn't really matter when their mindset on things have been changing all along the story, Hell Lacey is just about there in Act 3, after hearing Veronica talk about wanting a big family, she seem to like this idea, so she obviously is getting there, the MC will probably be there by the end of Act 4 after having a major mental breakdown as we can expect at this point for him to have.
As they say the road to Hell is paved with good intentions and its quite possible that by Act 5 the two of them will have progressed from being willing to go for a 'big family' to being able to accept the idea of each others fucking other people, allowing for NTS to be justifiable within the story.
Look if you go in any stories with the notion that the characters are going to remain static from start to the end, well you missed the point of having a story. If you say its some fundamental aspect of them, sure maybe at the start, and its why they are undergoing significant emotional upheavels, as a force that cause a change in their personality, you ignore all those stories where the character start one way and end the opposite way by the end, its very common for the coward or shy to become brave its a very typical idea. Do you think this happen just because of the character having self reflection? Or because the narrative force the MC to face a reality they aren't able to handle and which they are driven by external forces to change to be able to deal with?
Now don't worry if you are incapable of handling anything except pure vanilla, you'll probably get something passable as an ending and you'll be able to ignore everything else, an ending which ignore much of the story's character development and try to revert everything back to a shadow of what it was at the start, likely with neither characters quite satisfied because the narrative will have changed them to not quite fit that mold anymore.