For now she seems content to trust his words, she didn't project any kind of rebellion or doubt or lie when she agreed. Unless Ocean decided otherwise I fully believe she'd dropped it, especially since she herself expressed many times how Vanessa disgusts her and how she doesn't want to have anything to do with her anymore.
I think Nika and Bella are slowly creating a good relationship. It was incredibly rocky and hostile at the start, but they've built a lot of trust, connection and balance since, something that has been talked about by most of their closest friends (especially Bella's friends have been vocal in their surprise over how close they've gotten so fast) and family. Nika has slipped behind Bella's defences and I would say she will obey him when he displays such conviction in the truth of his belief as he did this time. Personally I was blown away by her unquestioning agreement, something that demonstrated an immense amount of trust in him on her part.
No, it's not too soon. Would you say it's too soon to make bold statements about someone who's killed someone else? Exaggeration, but Vanessa has substantiated her behaviour by what she'd had Mario do. Unless he's in the closet, I don't personally know any straight man that will put a plug in his ass, so that on it's own demonstrates a shitload of control and thrall. Also remember that she stated one of the ways she keeps control over him is to dangle the sex-carrot in front of him, but confirmed she has no plan to actually have sex with him in the foreseeable future and they've been together for a year. A year with no sex and some petting at most at their age...
She's also not all bluster, her behaviour is testament to her, plus the way she easily enraptured Nami and perfectly manipulated her.
Mario is her property, her plaything. Of course she'll defend him. Despots always defend what belongs to them, even slave masters defended their slaves from being stolen or hurt by others without their permission. There's also been 10 years minimum of development between the mother cheating and present time. People change. Her father is authority figure who has control over her as a father and someone on whom she's completely dependent (she stated she has no plans to ever work for a living).
She doesn't see Nika equal yet, he's still beneath her, but she is getting interested as a scientist would in a new specimen.
You can see what she is doing to her boyfriend as a "defiant behaviour and bold words", you're free to do so; but to me that is a sign of much worse beneath the surface.
