1. Achievements are not explained anywhere, and they're basically like a secret mechanic. They really could use some explanation. I remember that I only realized they're important when I found wiki page about them.
Replayability is always an issue with this kind of games. Basically they all are the same. You fight someone, when you beat them, you continue your journey. When you don't, you get fucked, which also is a reward in a way. But if you get fucked a lot, you lose the game. There's not much you can change about it. Some games have a plot that gives you a bit more motivation, but in this type of game, where nothing is linear, it can't be implemented.
2. Yeah, I also think that we need an automatic tool that shows you a list of arts and scenarios you still didn't see with the character. The only thing that helps you right now is a thingy on Progress screen that shows you random scenes you can check out. But that's too annoying to use as a main tool.
3. Random event on Progress screen is a name of scene you never saw in the game and can find.
Sex skills are just to bash them in combat and imagine that you have sex.
Well, there's Distracted effect, that you get when your anus is irritated after sex. Means losing Perception. There also is a Agility loss when your stomach is full of semen or eggs. And finally you can lose Strength and Agility when you lose your HP, the less you have, the worse is penalty. And inside combat you get penalties to Strength when losing a lot of stamina.