As for the discussion of Jenny: I am 100 percent fine with her not really changing all that much, because that is the most boring trope of sex games: Characters being "fixed" by the protagonist's magic dick. Sure, in this case our protagonist has one of those, but it doesn't completely rewrite a character from being their own personality to being more akin to a sex doll. Jenny isn't going to change because Jenny is Jenny. If you go into a relationship of any kind hoping to change somebody else's personality to something more to your liking, especially just based on how awesome you think you are, you're going to be in for a disappointment. And if you start a relationship with Jenny despite actively hating her and not seeing anything redeemable about her, then why are you really bothering?
One thing that I like about Summertime Saga is that characters always have a reason for getting involved with the main character in a way that fits their personality (for the most part, some storylines are stronger than others, of course). That reason usually isn't "I want to be different from how I am" unless they actively hate what they are, which Jenny, well, doesn't.