How do they know Dominic is being tortured?

Sure, the king is evil, but he also has a plan that required Dominic who gave himself up to save his friends. There is no reason to believe the king needs to torture Dominic, they don't even know what he wants from him!

They start by raising the possibility Dominic is tortured, but they seem to leave the"if" part very quickly, and base entire theories and plans on the question "how long until a mind break", and even assume that this specific reason was the cause of the destruction of an entire race.

It is as if they somehow read the comic...