I believe that Connie's acceptance of Carlo's death may have been tolerated, but based on the timing of when she was told by Michael. She was certainly not ready for it at the end of GFI. Only toward the end of GFII does Connie feel that she can have a relationship again with Michael. Perhaps the older, wiser(?) Connie of GFIII may have accepted that Carlo needed to die for setting up Sonny. I think deep down that Connie knew Carlo was a scumbag, but she was too insecure to leave him. Then again, Connie may not have ever accepted the explanation because she still believed that Fredo had "drowned." At least that's what she told Michael out loud, maybe deep down she knew the truth.