You can unlock panty hunting late in the game - it's not unlocked through direct progress with the girls themselves. Each girl has a set of several panties; the black panties are just one of the ones for that particular girl.
I wish there was a way to turn off the x-ray for the sex scenes. I really liked the character art but I don't find the interior passages of the human body sexy. And for some reason the artists decided to depict the vaginal canal as containing moving bumps that I guess were meant to be cilia but...
Once you start having sex with characters, you can get interactive sex scenes on demand. It's always pixel art, though. And you're right that the main plot progression is basically linear.
You need to go to your room (top right) and go to sleep. It's confusing because the interface leads you to think you need to click on faces, but you can click on rooms as well.
Lots of things, really. Basically anything that would make it feel like it's a different game. Whenever that happens, 99% of the time the newly updated game isn't one I want to play.