Its not that long of a game. When you know what to do its basically a breeze. That being said I still can't figure out how to get scenes: 7, 16, 17, and 18.
I managed to unlock everything, myself (I'm not sure why, given my general ambivalence towards the content in this, and I did it without translation help, so the comments on difficulty here are a little surprising), and I can share a save if needed, but I'd assumed the one provided on the last page already had everything. If it doesn't, someone let me know. Anyway, if you want to know how to get the ones you're missing...
7: Unlocked by completing the true ending; I believe you need to go back to all of the seals and interact with them to make them blue again, then speak to the older sister and go fight her at the final area.
16: Seduce one of the guards in town; I want to say there are two different scenes, depending on which guard you seduce.
17: Again, seduce one of the guards; that, or it was one of the other villagers.
18: Yet another seduction scene with the people in town.
I'm not sure what triggered the heroine's ability to seduce NPCs, but I'm sure it has something to do with having enough sex. You'll know it's possible if you talk to certain NPCs in town and it starts to give you two options instead of finishing the conversation.
Oh, and a little tip for people who may be blindly playing it and hate backtracking: pressing 'S' pulls up a menu that lets you teleport to most locations you've been to already.