No attempt should be made to "reconcile" Yahweh's hardening of Pharaoh's heart (plagues 6,8,9,10) with statements in the other plagues that Pharaoh hardened his own heart. The tension cannot be resolved in a facile manner by suggesting, for example, that Pharaoh has already demonstrated his recalcitrance, so Yahweh merely helps the process along, or that he is doing what Pharaoh would have done on his own anyway. Rather, 9:12 is a striking reminder of what God has been trying to teach Moses and Israel since the beginning of the Exodus episode: He is in complete control. However Pharaoh might have reacted is given the chance is not brought into the discussion. He is not even given that chance. Yahweh hardens his heart. It is best to allow the tension of the text to remain.