Part: Here's what I was thinking motivated Connie to act the way she did in this scene--

She's accepts her role and Michael's role in the family. She accepts Fredo's death as something that had to happen, whether by Michael's hand or by fate. She herself is not sure if she belives Mike's story or not, but realizes that either way "what's done is done", and so she accepts it, knowing that there is nothing she can do about it anyway.