Thank you very much. I'm glad you enjoyed it. In answer to your questions.
1) The buttons are your standard quick menu display. In order from left to right, top to bottom, they are: Back, Skip, Auto, History, Settings, Main Menu.
2) The bgs are mainly just inspired by japanese single bedroom apartments. Some of the decorations are there just to add a little life to the background and make it feel like someone's apartment.
3) I plan on doing more VNs in the future but whether they have are 18+ or not, I haven't decided yet.
4) https://incompetech.com/ You have heard them before. Kevin MacLeod is a very popular royalty free artist.
The game is on Steam as well. It was released Apr 14 so you should be able to find it if you have it set to show you 18+ content.
I hope these helped answer some of your questions.