I agree with Apple that Kay had made up her mind to leave Michael before she appeared at the Senate hearing. Perhaps she felt that her presence there was the duty of a "dutiful wife" (standard thinking in that era); and/or that it might help assuage her conscience about taking the kids and leaving Michael. (By current standards, he should have been the one with a guilty conscience, but in those days, the burden of keeping a marriage going fell on the wife.)

Kay almost certainly knew, or strongly suspected, that Michael had whacked Sollozzo and McCluskey. Her last look at the end of GF showed that she realized he really did kill Carlo and all the others on the day of the Great Massacre of 1955. Surely she would have realized by then, if not before, that he'd done the job on S&M. In the novel, when Kay visits the compound to leave a letter for (the exiled) Michael, Mama Corleone just about tells her that Michael did the job on S&M--she realizes it right then and there.