agree, he will treat her the same way he treats her in the brothel, maybe he will be even more perverted with some cosplays and fetishes... the best option for her right now is the shy fatso. plus she needs to tell Chiba that his feelings are completely one sided, even if it means less money now, simply because it will cause less problems in the future.
That's what I mean by fixing him (you can't fix people, but that's the best phrase I can come up with). If she can get him to understand how her life is from her perspective without including his selfish desires, then she can probably like a noble or something with him. I don't mean her only chance at life is being his sex slave.
Also with the 8-grade syndrome thing, I mean the way he's acting after being isekai'd. A normal person would understand that you can die in that place. Just because you have "cheats" doesn't mean you are suddenly invincible. Yet he acts like it's a game or an anime. She's being way more realistic in trying to find the most "normal" life she can without causing trouble or getting involved with the wrong people. He needs to learn that mentality from her.
The thing is I don't really think she'd be able to change him. Exactly because he has those cheats he can treat it all like a game. And looking at how shrewd he was when dealing with the ranked matches or whatever it was, he'll never learn it's not a game. Everything speaks against him being able to change... However, the most important is if Haru could even be bothered to try it...
Otaku guy is her best chance at having a somewhat normal life. She's going to have to work a little to correct that stupid (8-grade syndrome) brain of his. But it seems like he actually cares about her. Plus he's strong enough to actually protect her, not just rich.