Well I'm gonna ask a few questions, give recommendations and feedback.
-First of all I would like to ask if the game is gonna be visual novel stype or at some point we will have a free mode about who to interact with and things like that.
The visual novel style is useful for new developers because they are in total control about what happens, it's easier to make and quite to the point, basically it doesn't have repetable events and is constantly advancing.
On the other hand we have the free mode style, the good points of it are that it gives the chance to make variations to events, more involvement from the player because how they act influence the other npc's and it's more complex in the sense that it needs more variables and stats (which is usually good for a player), on the other hand if badly done it makes the game grindy or even a waste of time.
Personally I think that based on the game fetishes, it makes more sense to be based on the free mode style, mainly because one of the funny things about corruption is for a character to react to the same thing differently, and in the visual novel style you usually don't have the same event multiple times, you can notice the corruption in more advanced scenes but it will never have an effect on the past scene.
-Optional fetishes: Here is my personal view of it, you have it or don't have it, rather than making it optional just make it avoidable, if you truly have to make a... "protection" against a certain fetish, do as many games do and ask at the start, if people say no to that fetish then just lock the start or even lock certain options in conversations. Personally I like to go along for the ride and see what happens, it pains me a lot when while playing I find for example an option that says (NTR PATH BE CAREFUL), I'm just like, "uh, now I know what will happen so... not gonna choose it", it just feels weird for me things like "force your lose for the female MC to be raped by the enemy", "choose to be cucked"...
I would prefer things to be avoidable rather than optional, avoidable means that under certain conditions something happens or not, but this is just my personal view over it. However be aware of something, making everything optional is a pain for both players that want to find x content and the developer, it is only good for those that don't like that content.
This is just my opinnion but if you like swinging then just add what you like into the game and even more, dont make it optional. Even if you want to not make it forced then just make it avoidable, for example a non corrupted girlfriend wouldn't be willing to engage in an act such as swinging, but if she is very corrupted perhaps she would even propose it herself.
-Basically think hard about what you want to do, swinging and NTR are kind of similar in a certain sense and swinging the dangerous part is that it can develop into NTR.
-One thing that actually always make me wonder is that sometimes a character may make a mistake, but many creators make things so that "one mistake you are in the bad route", it's kind of weird because people make mistakes, I find perfectly normal that the MC would check Olivia's "goods", however if he is truthful towards his wife then nothing happens, I quite liked that just because a character did something "bad" it isn't the end of the world, however I would find that my girlfriend became wet by showing herself to a stranger like a "big warning notice", even more with a bad influence near her that had sex with men and women and it's a manipulative person.
-Finally to end, I hope that you make Olivia truly manipulative, something interesting that could happen would be that Olivia is corrupting your girlfriend and then to push for more she convinces her about having a threesome between her, your girlfriend and you. The funny thing about having female NTR based is that we can have profits within that danger, perhaps Olivia would fall in love with the MC or she will rather use the threesomes to push the girlfriend more in the sense that "they have had sex together with the same man, so why can't they both have fun between themselves without the man?", it's quite a manipulative argument and it seems likely to succeed, after they start to have fun with each other it's easy to develop a full route based on it.