Yo good catch with the Ratkin and Bareshade guy scene totally forgot about him, also couldn't remember if the ratkin could spread it or just the big rats.
You are right about the werewolf and shapeshifter language, I just thought since clearly the Nameless isn't from Aigran (name of the world(started a new playthrough because of this)) and sees a potential ally in us that he was using some sort of trikery to make us think we could trust him so we would further his goals or something.
About the suspects... while I grow more suspicious of Bernard and what he's doing I don't believe he could be the shapeshifter because he seemed to know our mentor at the start of the game, Hayden being the newest member to bareshade and his odd colour of hair (I know he explains it but I'm not sure i buy it) is definitley suspect but I don't know much about the swamp creatures, what they can do, or their origins.
But with Feliar I don't know I feel like having liar in his name and someone writing "Feliar is a liar" is probably a red herring? Maybe Victoria or someone else did write that I don't know maybe he lied to Victoria and that is what caused their falling out and maybe she did that to get back at him?