The 9/11 commission should not accept Condi Rice's offer to make more appearances in camera, but not under oath. If she wants to rebut a witness who has testified under oath, she should be willing to take the oath as well. Her claims that it somehow violates separation of powers for her to appear are nonsense, as two previous NSC chiefs have testified before Congress under oath (Berger and Brzezinski).
Besides, she owes an explanation to the American people, not just to the commission.