I still suspect the sommelier. One well-known method in Japanese blogs for uncorking wine was thunking the bottom against a hard surface (e.g the wall). This will push out the cork without making any visible marks. He might have used the hammer to hit the bottom of the wine bottle instead of hitting the bottle against surfaces because that would make noise. He also has that opened eraser, and I'm guessing he probably used that eraser as padding so that the hammer won't break the bottle. This is usually not advised because shaking wine would mess it up somewhat, but since it was made clear that it's not actual wine he probably won't care much.
As for where the poison is, I'm thinking it's in the soap container because the hint is something to do with light. Due to "index of refraction", sometimes
transparent objects can turn invisible inside liquid. People won't think to check because it clearly looks like there's nothing there (especially because the plastic pipe of the soap spray is visible), so maybe if the poison bottle is super small and made of glass/acrylic or something it can be put inside and just become invisible.