not to have a go at you but your #2 is BS
NTR just means stolen from you, there doesnt need to be a monogamous or even any sexual relationship for it to be NTR or 2/3th of all NTR would stop being NTR, Mothers are getting stolen, Childhood friends getiing stolen, because MC is too dense to see that she loves him etc etc. Not a single one of those scenarios needs a sexual relationship just that the Protag knows the girl.
MC can even have sex with Chika before he knows shes a whore and even if they have sex, she still sleeps with her customer later and gets found out by MC by pure chance.
yeah thats not correct, NTR or netorare is a Japanese word that directly translates to "to be stolen" (you have that part right) or "to be cheated on" and it elusively refers to a scenario where a character's partner is 'stolen' by another person, usually sexually or in other more PG cases romantically a person you are not romantically or even sexually involved with fucking other people is not NTR neither is Mothers/childhood friends getting stolen because there is no sexual or romantic feelings shared by the 2 characters. but you are correct I completely forgot about her being seen leaving a hotel room with another guy, my bad in that case it is in fact NTR.