Interrogating these black-robed magic girls, to be honest, really isn't challenging at all.
Although they're magic girls, at the end of the day, they're just a group of ordinary girls. Even if they can fight witches, it doesn't mean they can withstand interrogation. In fact, most of them are so scared just by standing in the interrogation room that they're almost about to wet their pants.
However, this doesn't mean Duanmu Huai will show them any mercy. Basically, he'll ask each of them the same questions: name, age, school, why they contracted with QB to become a magic girl, what they're currently doing, what Magius's Wing is as an organization...…
Although it took a lot of time, Duanmu Huai at least managed to get a clear picture of the general situation.
"Alright, I know you're all very tired, but now is not the time to sleep."