-Bodolf is the basement of one of the ruined houses. You get a message when you enter it if you have gathered enough clues that one of the walls seems less dusty than the rest, which reveals a secret entrance to his mini-dungeon.
-Which NPCs have no interaction?
-The volcano has a second floor (and will have even more in future updates) with a giant black hexagram at the end of it, and a smaller, red hexagram in front of it. Step on it and you will fight the Gatekeeper.
-I have in fact added a quest log. All new quests use it, and I'm slowly adding it to old quests as well.
-Floor 13 is the top you can reach in the current version :P Floor 14 is Theodoxa, and I haven't completed it yet. Sorry to say, you've done most of the currently available content.
Honestly I'm trying my hardest to cross things off the list I have to do to release the next version, but the list is just huge (well, usually it starts with little things, like just changing the face graphic used in one scene, then I decide I am not satisfied with the dialogue so I remake it, then I decide to add more options and potential outcomes and so on). Every day I make progress though.