For the dev, don't let people dictate what you do or do not put in your game. Its your game, not theirs. I see some people are using the "if a girl does anything with any guy that isn't MC, then its NTR" argument but thats not true. They're just being overly sensitive, and I say that as someone who dislikes NTR. Teasing and flirting aren't NTR, its teasing and flirting. I see comments about a girl sitting in someones lap because there isn't enough space in the car....well what are they supposed to do? Hang on to the roof? Thats not NTR. This game seems to have a mom/dad character, if they do something together its not NTR.
I know I may have a more strict definition of NTR, but the thing is you can be jealous (a key attribute of NTR) and it NOT be NTR. A part to NTR, besides jealously, is the loss of an LI IMO. If she wasn't yours to begin with or hasn't shown any indication of being an LI, its not NTR IMO.
Things that aren't NTR IMO.
1- A girl doing things with her boyfriend, husband etc. They're in a relationship, what do you expect? Can you be jealous? Sure, but its not NTR they're doing what people in a relationship do.
2- Flirting and teasing. Maybe thats the girls personality? The only way this can be NTR is if the girl is in a relationship with MC, if she isn't (aka she is single) then it doesn't count.
3- Dream scenes. Its a dream, it didn't actually happen.
4- Side-girls doing stuff. They're aren't LI's.
5- Sharing/Swinging aren't NTR. If you voluntarily share a girl or swing with her, you obviously aren't jealous which defeats the point of NTR.
As for vanilla and NTR paths, just make things optional and it should be good enough. Although I will say that hardly any game on here manage to have both vanilla and NTR paths co-exist and do it well. Those genres attract the opposite players, so its probably better to just commit to one or the other.