Fucking bravo mate. I don't bother replying or arguing to posts on NTR on any forum cause it just isn't worth it, but I might link this page in the future as a lazy way of doing that.
I will add one common scenario (a trope that would seek to explain or soften the blow to use your own words) that I see in many games and often stirs debate about what is and isn't NTR. And that is when both you IRL and player character in game know from the start that the object of your lust is a crazy huge slut. Queen Opala is a great example. Farah (the mother queen) fucks anything that moves, including animals. For gods sake she gave birth to a horse. A 'hard' rape scene is rather difficult to conjure with a character like this, unless its someone she despises for plot reasons. And yet you will also find people decrying these games as full of 'cuckoldry' and 'ntr' because many of the erotic scenes involve some demonic/animal/human 3rd party, rather than the player character himself fucking them.
Thanks for the kind words. NTR a niche subject that stokes visceral anger in many due to either experience with or anxiety about the feelings the work is attempting to induce. This is probably why some of the discourse is both exhausting and simplistic as I'm sure you've seen first hand.
I think that the scenario you describe could be a framework for something like NTR and I would say it has the potential to be more in line with my take than "softening the blow" but could also not be NTR at all. It comes down to the disposition/actions of the player character.
If what you describe is from the point of view of a protagonist who IS jealous of the slut in question and IS threatened by the level of sexual deviance going on without him/her AND/OR tries to stop it and fails OR isn't really together with the slut character but WANTS to be and is trying to either seduce or control or "protect" that character from being a slut but failing... that could be NTR and it could even be really good NTR because, again, it's the agency of the sexual character that is driving the conflict and the protagonist character is in the position of attempting to prevent something but being ultimately powerless to do so.
On the other hand, if the point of view character doesn't care, isn't jealous, likes the behavior, and/or demands it, we're not talking about NTR anymore. However, I think the passions run high enough that people get triggered from things like that, perhaps by injecting their own jealousy even though it isn't voiced by the player character and/or interpreting the unavailability of the content in the game as a lack of control. The game might not be NTR, but the triggered player may make up the difference in their mind. Doesn't mean that they should be listened to or taken seriously. Those guys who can't see non-access to an object of desire without experiencing NTR probably have their own shit to deal with. A lot of it.
Most notable were all those idiots who decried the "public" content in Akabur's Princess Trainer and Witch Trainer games, eventually causing Akabur to make public content optional going forward. However, none of that public content was NTR or even cheating since the protagonist character demanded that Jasmine and Hermione go out and have sex with others. It's an extension of the dominance and control the protagonist is exerting over the object of desire, not the object utilizing their own agency to thwart the protagonist's control/dominance. Jasmine and Hermione's agency is gone and that's why they are having sex with whomsoever the protagonist tells them to.
Another example closer to your original was Azalea the stop girl in PT Gold. Even in the route where she had sex with all her customers regularly, her doing so was at the protagonist's direction and the protagonist was never denied sexual access to Azalea due to her own agency/choosiness/standards/desires. Had the scenes been written (and maybe there were dialogue routes that I don't recall now) where the protagonist, after the fact, tried to maintain exclusive sexual access to Azalea and failed or was rebuffed or ignored, that might be NTR depending on the protagonist's reaction to that.
tl;dr I think the distilled heart of NTR is about jealously, lack of control, and the agency of a desired sexual partner. Betrayal or humiliation are often in the mix as well though neiter are necessarily a requirement.
Those three things can be expressed in different kinds of scenarios, and I think the character already being "corrupted" as you point out can qualify as NTR still (and not even diluted NTR where the agency has been compromised) but if the writer makes an attempt to weaken or do away with any of those three things, it becomes less or not NTR.
PT Jasmine is not NTR because she has no agency, the genie controls her, and the genie never expresses jealousy towards jasmine when she's being whored out. When a girl is blackmailed or raped or drugged, it's less NTR because agency has been weakened/eliminated. When a character is a slut already, you could still have an NTR scenario if the point of view character is jealous and fails to control the object of desire because the whole point of her being a slut is that she is exercising her agency.
Agency + Failure or Inability to Control + Jealousy at Result = NTR
That's the base story engine and you can mess with the relative values, but it'll still be NTR if it has all those things and you can attach anything else to it or put it in any context and it will still be NTR... but if you get rid one or more of those three, you're probably doing something else.