the last page is kaneki in the prison, you can see the levels, so he had to have opened them. At least it looks like the prison because before that we saw him in a locker room.
I think you have to take the ghoul rankings from the lower tier of investigators. Arima is in a class of his own compared to everyone else. So what might be a SS ghoul to an A rank investigator, could be like an A rank to Arima. Look how Matsumae killed Ihei who was thought to be hot shit...
