What is the significance of the fact that Hashem once again hardened Pharaoh's heart?
Ramban: Following Makas Bechoros, Pharaoh was afraid of Yisrael, and had not the least intention of chasing after them - even assuming they would flee, 1 in which case Moshe was free to do with them as he pleased. That explains why Hashem had to harden his heart, and decide to chase after them. 2
And not return after three days, as promised.
In fact, Hashem hardened Pharaoh's heart one more time (See Pasuk 17), when the sea split and he beheld the awesome sight of Yisrael passing through the Yam-suf on dry land. He would not have dared follow them, had Hashem not hardened his heart (Ramban).
What did Hashem mean when He said "ve'Ikavdah be'Pharaoh ... "?
Rashi: When Hashem takes revenge of the Resha'im, He becomes aggrandized and honored. 1
Why does Hashem make a point of mentioning first Pharaoh and then his army?
Rashi: Because, since he was the first to sin, he would be the first to be punished.
Why does the Torah see fit to inform us that Yisrael turned back and encamped by the sea, as Hashem had commanded them?
Rashi: In order to demonstrate their praiseworthiness, in that they went back towards Egypt, and did not say 'How can we go back towards our pursuers?' 1
But rather they stated categorically 'We will only do what Moshe tells us' (Rashi).
Rashi writes that the Torah teaches the praise of Yisrael, that they accepted to do what Moshe says. They complained "were there not graves in Egypt [that you needed to take us here to die]?!"
Moshav Zekenim citing R. Moshe: The Erev Rav complained.
Moshav Zekenim: A Midrash says that they saw "Mitzrayim Nose'a Achareihem", i.e. the angel of Egypt coming to save Egypt