Shouko's the most suspicious one at this point.
Supplying the investigation with a specific time window is extremely suspect behavior, first off. She brought something cold, which she could have been planning to place on the stove herself (in order to "prove" the window of opportunity she gave them) if Mina hadn't put the bags on it first. As with plenty of multiple-victim murder mysteries, the shogi board legs were probably used to disguise the real order that the victims were killed in.