Kinzo's presence in the dining hall

Axioms

• All of those present at the family conference acknowledged the presence of Kinzo!
• No person would mistake Ushiromiya Kinzo by sight. No matter what disguise might be used, they would not mistake Ushiromiya Kinzo by sight!
• Kinzo's life or death status is the same at the start of all four games. The setup was not different for the fourth game alone...!
• Kinzo is already dead at the starting time for all games!
• Before now, I have proclaimed that no more then 18 humans exist on this island. I will now lower that by one for Kinzo!!
• No more then 17 humans exist on this island!! That excludes any 18th person. In short, this 18th person X does not exist!! This applies to all games!!!

Corollaries

They all acknowledged Kinzo because it's in the terms of the deal Krauss agreed to. In exchange for a hefty sum of money they would all testify that their father is alive and well. It's because of this framing that, from our perspective in Purgatory, we are actually shown Kinzo coming down and interacting with everybody.

Kinzo is always dead from the start because we think X Beatrice shot him some time before the family conference.
The presence of X as an 18th person is explained because Shannon and Kanon are two characters played by Sayo, who only counts as one human.