Pretty sure it's Nishino. Looking at the setup of the boxes, what Mouri seemed to notice was that they were set up in a way that one of them stands out. You remove the one that stands out, and another one still pokes out. It makes it significantly more likely to draw each of those. (It's like a variant on the baumkuchen illusion one.)
On the previous one, looks like Wada was given the middle piece of the 9, and feeling along the edges would let him easily determine which was Wada's.
I'm thinking the guy who died might have liked magic tricks of a sort.