I don't like mechanics where you only get NTR if MC is an idiot or ignores her from time to time. Why should I play such an idiot? He is clearly in love with her and this should be reflected on the NTR path, too. Actually, it is what makes NTR interesting in the first place: if there is no connection between MC and LI, it's not NTR, it's just a woman having sex with another guy. If they are in a loving, healthy relationship, having an antagonist who "owns" the LI adds the desired drama and tension.
Take for example the mother-NTR: if it's really about "She cheats with Matt because she is lonely / her husband is away often", that's kind of boring. It's not impactful because the relationship between mom and dad is already fading and Matt just occupies the "empty space". NTR is imho more impactful if there is no empty space to be occupied, but already occupied space corrupted and taken away: so the mom is not bored or horny, but still serves Matt because he e.g. dominates her in a way her husband doesn't and can't do. She wears slutty clothes because Matt tells her to, not because she feels better with it or whatever. She lets Matt fuck her, but denies her husband because Matt said so- not because she doesn't want to or finds Matt more interesting. So NTR, imho, is not about grabbing space somebody else left behind, but about owning space that is already somebodies elses.
And, as a last remark: I personally don't find "behind the back" cheating and "lying bulls" appealing: so if Matt really got a blowjob, he should have just told it right away without any fear of MC. It shouldn't bother him that MC knows, and he maybe could have even made him watch. This obviously should be important for the Ava-NTR, too: don't cheat behind the back, make it clear that the antagonist owns the girlfriend and is not afraid of MC.