Mizuho (the surviving girl from the previous manga) killed herself, according to Touma's story, which is why she stopped contacting Ryosuke (previous manga's MC). Minami is a completely different person.
Someone at the company is definitely helping this Kiriko. The warehouse's inner door lock was broken (Kuzehara didn't even know it was broken, and it was implied that he's abused other women in there), and someone (probably Touma) opened it from the outside and shouted, to get Minami out of the room and lock Kuzehara in there to die.
e: Oh right, I forgot about that cop. Yea, he is definitely the one working with Kiriko, then.