As for NTR conversations - I'm not a fan of it personally - I understand kinks for each are different so there's no judgement one way or other.
I think the game here doesn't need a bunch of it to be successful.
On the other hand, I'm not opposed to letting some exist if it's avoidable. Alexis scenes were avoidable. If you're playing the game and you're paying attention, you should be able to pick up on the clues that the dude she was flirting with at the table, and knowing your previous run with her ended due to her cheating - it's fairly obvious that was gonna happen if you didn't speak up.
I think there are some characters who make sense with - others do not. The definitely NO include: Bree, Hanna, Lavish, Shiori, Kylie, Cassidy (once you fix her mean daddy issue and she is dedicated to you). Samantha (if she left Ryan due to cheating to be with you), Palla if you're treating her well and helping her with manager etc.
I would like to add Lexi to that as well - I know she's a slut when you meet her, but there's some story and dialogue setting precedence that she sees MC as her prince that finally saved her from that life and she is spun on MC. So it's a special feeling kink to think you've tamed her from wanting to be a slut to only wanting to be with MC.
Those who make sense: Alexis, Sasha (if you cater to her 3some idea), Samantha (if you cater to her 3some idea), Audrey (is a major slut), Palla (based on story), Aletta.
Kleio, Anna, Morgan with each other.
To implement, it would be worth considering for some characters (Audrey, Aletta, Palla, Sasha, Samantha, Alexis) to add a new "points" mechanic like the LPs and GPs, called "Cuck Points" That way - the MC player has full control over avoiding or allowing and for whom.
BUT.....since it's safe to say MOST people prefer to avoid or not interested in NTR, it's worth pointing out that by adding this content, its using time/resources of the developer to cater to a smaller group of fans and taking away that time adding other content more appreciated by the general public. SO if it was to be utilized, the hope and realization is that it should be very little at a time and for those who are fans to not expect fast additions and developments because that simply isn't good sound choices on the Dev's part to cater a lot to it.
The thing is that many characters have past relationships that should be a point of conflict even when the girl is in love until you can intimidate, talk or do some other action to stop it with the rival.
We have for example Scottie and Sasha, overall even if we said to Sasha to dump that guy for good that doesn't mean that it would be the end, Scottie doesn't seem the type to say "okay, as you say I will never come back". However when you have enough fitness you could give him a lesson so he stops bothering Sasha.
Same thing with Ryan, overall he wouldn't give up on Samantha unless... we actually solve our conflict, overall a reasonable one would be that he doesn't truly loved Samantha but rather he was overcome with envy, he admired us and wanted to be better at something at least, then Samantha appeared and saw it as a chance, that's why he likes to show off in that sense towards us.
Aletta and Dwayne is another conflict and one that could even cost us our job. Overall seeing the situation Dwayne is in perhaps the best way to solve it would be to manage to have him kicked out of the company.
Overall perhaps the best way would be that "relationship conflicts" would just be left for hard mode or perhaps add a hard EX mode, but personally I just feel that those conflicts if they don't happen it would make incomplete many relationships.
Then we have Alexis extreme example, overall that's a pure cuckold relationship when you don't stop her (albeit I personally don't like the way we stop her but rather I would have liked to make her more submissive and dedicated to us until she doesn't have eyes for other guys).
And we have too all those lesbian relationships and other slutty girls.
I do agree that some girls don't need those type of conflicts, out of your list for example Hanna it would feel a bit weird to add one for her, Lavish the point feels like it would be more on dominant/submissive relationships and that at some point she will want to end up being your boss (she is really ambitious), Kylie is a yandere, overall I would want more to see actions related to it and be able to accept being part of the harem compared to NTR stuff, Cassidy feels the same as Lavish, with a focus on dom/sub relationships.
On the other hand I would take out of your definite no for sure Bree, she is the female MC and she will have a lot of sex adventures compared to any other character, I would expect that unless we tame her she could end up going out with other girls or boys. In the case of Shiori... she has a second job and rather big problems on her back, so I wouldn't add her in a definite no list.
I just think that we have on one hand the relationship with the girl while on the other the conflict with the characters that go for that girl, if you solve the conflict for good you can't be NTRed, on another point I don't take in this concept NTR as a final thing, but rather that you could seduce back the girl.
Just a tiny list of examples of what I mean:
-Scottie x Sasha: To stop Scottie you have to beat him in 3 fights, the first needs fitness > 25 (15 if you have martial arts), the second fitness > 50 (35 if you have martial arts), third fitness > 75 (50 if you have martial arts). After you have beaten him enough he will give up.
-Ryan x Samantha: To stop Ryan from trying to recover Samantha you will have to try to solve your past relationship problems with Ryan, different from the relationship with girls he would start at -100, being hostile, and you would have to try to fix your relationship, learning the reason why he cheated and having him admit that he never loved Samantha but rather he felt envy for you in the past and one thing leads to another, in the end he was just with Samantha so that he felt he was better than you at something.
-Aletta x Dwayne: You will have to end up blackmailing Dwayne or kicking him out of the company, basically search for things you can use to accomplish it.
-Anna x Gewndoline: You will need quite a few serious talks with Gewndoline to make her give up, basically it will require a few charm checks, similar to Scottie issue after the 3rd check she will give up on Anna... but just because she is now interested in you.
-Morgan x Kleio: Different from other conflicts you will be able to solve it more easily, just make Morgan less interested in girls until she leaves Kleio alone.
Even more there are possible additional relationships that may happen (mainly lesbian ones that could develop in the game) and ones that could happen in the future (Minami and a boy in school for example).
I just feel that the game would be incomplete without those additional conflicts. Of course the game could let them exist and give the player the option to lock all relationship changes cause of them (overall that the "rivals" couldn't seduce the girl no matter how much time passes).
Sorry if it was a big post but wanted to explain my personal point of view.