Idk, if I dated someone and this girl was asking all the intimate details about me from my relatives or friends without me being there or without asking me first, that would be a serious red flag for me. Trust goes both ways. If you're so paranoid about your partner being a cheater that you can't trust them to have boundaries (whether to talk or not to talk about one's ex is a personal choice that depends on willingness to open up) and need to constantly sniff around their secrets to calm down, I wouldn't trust that person because clearly there are some personal issues with self-esteem that I can't really help with, so better to spare everyone from disappointment and move separate ways.
The fact is, you can't control whether a person is a potential cheater or not (unless they have that reputation or something). No amount of controlling will help with that. The best tactics to avoid cheating is to be a good partner and show trust. If they have a good time with you, they won't want to risk it for some fresh excitement. And even if they do end up cheating, it's not like it's the end of the world. Not all relationships are meant to be.
So basically don't be like Wade or Axel, do the best thing to keep the initial spark that helped the relationship bloom in the first place, like Ian being supportive of Cindy's self-discovery, as well as not giving up on personal aspirations that Cindy is empowered by. Keep things fresh and exciting, and she won't want to leave or cheat with the first self-absorbed playboy. Sure, Axel may scratch her ego and uplift her modeling aspirations, but does he care about her emotions and well-being? Does he care about her as a person? Not really. And that's why Ian will be more valuable to her than any Axel, because he can read her like an open book and help her become the best version of herself with his careful and beautiful writing.