In the novel (sorry, I can't remember), does Mama actually tell Kay that Michael killed two people and is in hiding? I thought she just told her that she should forget about Michael; he is not the man she thinks he is and a life with him is not suited to her; it would only cause her pain.
If I recall correctly, the only close-up on Kay's face during the Senate hearing is during the question posed to Michael about McCluskey and Sollozzo in 1947. It's the only time we see a reaction from her. I thought that was done deliberately to show how the truth of Michael's absence was finally dawning on her.
If Kay knew that Michael murdered two people, one of whom was a police officer, and she married him anyway without asking about it, why was she so upset and in need of an answer when Connie accused Michael of having Carlo killed (even though Michael didn't carry out the actual killing himself)?