Mc's gf already fucked the other guy when mc was in hospital so it's ntr already but
Only if he caught them. That's an essential part of netorare. The main character must suffer from catching his girlfriend with someone else, where it does not necessarily be sex (would have been hard to do on the stages where the term was defined).
we the players didn't saw
It's not just about the player, NTR is mostly about the character. But of course, if you don't show it, it's not in there. Every woman in movies has nipples. But as long as you don't show them, the movie gets a lower rating.
What happens off screen, stays off screen.
I never cared much for Eva, she's just there (sometimes), but brings nothing to the story. But even if I did, I would not be disappointed. The last event was the first interesting part told about her.