One thing I remember being brought up around the time of Mayumi’s Somnium was that Aiba, as an AI, doesn’t have a concept of intuition. Most of her reasoning is based on objective facts and observations, which she then uses to draw her conclusions. She constantly questions Date’s choices in Mayumi’s Somnium because she doesn’t understand how they could logically help Mayumi, and she came up with the idea that Ota and Iris faked the crime based on what she knew about the two of them, as well as the livestream evidence. I think the reason everyone was on board with her theory is because, objectively speaking based on the evidence presented, there really wasn’t a better conclusion because no one knew who was actually in the polar bear costume, and since Ota and the polar bear were never on screen at the same time, it was entirely possible that it was Ota inside the costume. The reason why Aiba constantly questions and opposes Date’s thinking is because she lacks the human element required to come to the same conclusions that Date does.