Yea! you are right. Haha i did not mention how was the boss killed. I just said he was killed. And the last person who went in would be the killer. So it doesnt matter how he was killed. Lols.
Since Gin gave iced wine to the boss, the fingerprints would not be clear. If the fingerprints were clear, it either means the fingerprints were left later when he did not drink iced wine.