'Preening' works for me here.
Preening is what a bird does to clean itself and make its feathers look beautiful. When we say preening about a person, it disparages someone who is proud of their appearance.
The mother in this passage was proud to say that she had no idea what could have led her child to become a murderer - surely, she felt, it had nothing to do with her. Then when she recalled having broken his arm when he was a child, she began to realise that she did carry some responsibility for his violence. In this context, I read "preening incomprehension" as "smug denial of responsibility".