I am not sure how the sex scene scoring stuff works entirely, but as you have more sex you get more types of interactions and it seems that the more types you do and the amount overall, the better the score, at least roughly from what I've haphazardly pieced together.
As for sexual interest, the dates give you access to the 'software store' or whatever that unlock erotic conversations and sexy games that you can talk to her about/play with her, and those increase her sexual interest slowly, it's a grind at the stage I am at so far which is about day 14ish.
The hentai stat is basically how horny your character is, peeping on her while at work, having the sex shop girl jack you off, and abstainting from sex to get the masturbation scene is the only ways I've found to increase it so far, it's a bit grindy but I've already hit about 55 or so by day 14ish without trying to focus on it too much.