Why does the Torah use the expression "Vayichbad Lev Pharaoh" here?
Seforno: Despite the fact that not one animal of Yisrael died was a phenomenon that belied the imagination,
Pharaoh exerted himself to overlook the consequences and refuse.
Hadar Zekenim: After the first five Makos, Pharaoh did not willingly detain Yisrael. Hashem forced him in order to show His ability.
As is borne out by his need to send spies to check whether any of Yisrael's animals had died.
Why does it say that nothing died from Yisrael's flock "Ad Echad"?
Malbim: One animal died, of a man born to a Yisraelis from an Egyptian. The Torah considers him to be an Egyptian, but Pharaoh thought
that he was a Yisrael. Therefore, he hardened his heart.
It is not clear whether Pharaoh did not know the Halachah, or he assumed that also his father was a Yisrael. (PF) The Pesikta says that there was only one such man, i.e. the Mekalel (Vayikra 24:10).