Question first: Does anybody know how to trigger all brothel scenes? I ended up with the meat toilet scene (it's like the stuck-in-the-wall scene) and now the owner throws Tsubaki out everytime. There is still an orb in the gallery in the same row as the other brothel scenes and it's name is translated to something like "Kunoichi bought with money". I have no idea what to do about it.
In Chapter 5 (After the red demon has been beaten) you should find the blue-haired girl in the amusement district. Eventually she will tell you about the water god in the shrine south of the hideout and how he might restore things dropped into the pool. It's possible that you need to trigger this hint, but I wouldn't know what may be required, Go to the shrine; above the bell and the little pier should be another set of bubbles. You might need to ring the bell; I ringed it way before and I didn't have to after I talked to Aoi. You get three options: The first two I haven't tried. Apparently they're about accepting a silver or a gold kunai. The bottom option is translated to "don't care about h-scene"; it will trigger an (rather good) h-scene with the slime and Tsubaki acquires the water god kunai afterwards. It's good, but certainly not the strongest kunai.
Regarding the old man with his arrow/cheat code message: You can later talk to him when he's cowered in the corner; just walk behind that divider. It may look that you can't, but you can. He's the harassment guy we might have met in dungeon 1. No idea what's up with the code.
The brothel cat scene can only be activcated after you watched both in-dungeon cat scenes: I missed the scene in dungeon 2 and turned every stone to find the missing scene (the owner gives you vague hints regarding which conditions might be fulfilled).