Ffffffff, I love you.
Yes, and yes. No, you won't have to buy anything separately, just when the game is released or as each route is, you'll have to go to your purchases page and download it.
By preordering, you bought the game early, so you won't have to purchase it again. However, the game isn't finished yet and currently is in beta.
I'm curious, will we be able to buy the OP song once the game releases?