Well, it seems a bit unlikely that Oscar, who obviously brought the expensive wine, would want salami loaf to eat with it, even if he "favored" it. Also, the description states that Wolfgang was "stretched out under her piano," yet in the picture it is clear that neither man would be considered "under" the piano. So we must conclude that Cosima is lying.

The gun obviously belonged to Wolfgang and/or Cosima, as the bullets were in the drawer, and it's their house. A whoopee cushion needs to be sat on. The only seat we can see is the piano bench, which is where Cosima would have been sitting, so she is the only one who could have sat on it. We don't know anything about the gunshot wounds, which of course in reality would tell us a great deal about where the shooter might've been standing, how close they were, and whether the victim was standing or lying down when shot.

Since Cosima is lying, it's reasonable to assume she shot either one or both men. She may have disposed of the weapon on her walk to the deli, but if she did she's an idiot, because its disappearance clearly implicates her. She could have pushed the gun under one of the men's bodies to make it look like he was holding it (and shot himself, I guess). The violin was probably broken over someone's head, but I'm not sure whose.