Her brother talks about how private she was. Her Facebook didn't show that but I think its because everything she shared, was just every day life. I really don't think Chris had any idea that Shanann had shared with all her friends, how he had been treating her and that he no longer wanted the baby. He thought they would still think of him as a "nice guy". I think that is why they all caught on so quickly. Besides that, they really knew Shanann.
I think Chris gave up caring about SW after the nut incident and his dad's reaction to it. The reason everyone caught on so quickly was because SW was telling how suddenly bizarre he was acting toward her and her pregnancy.
With a toxic narcissist, everything is about control. If he had agreed to go to therapy, the red flags would be thrown by the therapist. JMO