1.) What Midou has been doing so far is not the absolute worst that she could go to. IRL examples can be found which are just as (and often more) horrible.
2.) How her victims have responded is also not very far from reality; I know of a few cases where even the mere act of eating became difficult for the person traumatized. What is quite improbable with the narrative is how all of them have just about the same response, not asking anyone for help. (Then again, I'm no expert on Japanese society.)
3.) The imouto was clearly carrying the idiot ball (for the sake of the plot, I presume) when she went with Midou. Even when simply conducting an interview or a survey, you do not go alone, without support, with someone like that. (I remember our administrator doing regular checks on our positions and activities, even when we reported that we might take some time or need some privacy.)
4.) The MC has been carrying the idiot ball on and off for some time now, even considering his traumas. (He enlisted private investigators but couldn't contact law enforcement? Seriously?) Even after establishing his fake relationship with Midou, he took no course of action to alert anyone to his situation.