Okay, I don't actually know, but what makes the most sense to me is that torture is used to weaken mental defences so that Ren can get in more easily.

Again, it's unclear if it's necessary or not. Are people more or less susceptible to mind-reading? Can people improve their defences? How effective is torture at breaking defences? We just don't know.

Alternatively, they tried torture, it didn't work, someone decided it was worth calling in Ren despite the dangers associated with bothering him.