How many horses were really left in Egypt when Pharaoh with a hardened heart, lead the charge after the former slaves? How was he able to recruit all the chariots of Egypt, considering that the country was destroyed? How is this relevant to Na Nach?
There had been three pleagues against the "animals" of Egypt. The first one wiped out (EVERY SINGLE) domestic animal from the fields. The second one (EVERY SINGLE) domestic animal in the barns. The third one (EVERY SINGLE) WILD!! animal not captured and saved by 'good hearted (or hungry) Egyptians'. What could Pharaoh use to pull the chariots of the dead first born?
Gd heardened his heart by allowing his magic to produce horses out of any animal (including for example the rats they were raising for food..inspiration for the cinderella story?, think about the importance of the shoe in both the stories). Sure if you can create great stallions and turn all the chariots of Egypt into formidable battle wagons lead by three great powerful steeds, then of course you have a fearsome army and the slaves should be afraid!
Why did Gd "re-animate" a dead Egypt just to kill them again?
The midrash says that it was easy for Gd to take us out of Egypt physically but very hard to take Egypt out of us and that was what the re-destruction of Egypt was supposed to do.
What is crucial here is the word EVERY SINGLE because every single Hebrew was saved while every single Egyptian was destroyed. This is reflected in the later passage where Amalek is confronted from generation to generation until one day EVERY SINGLE Amalekite will dissapear. The hands of Moses would go down forcefully and Ahron and Hur paid a terrible price for keeping the hands of Moses up by force. They were supposed to be a beit Din and accept the will of Moses not CONTRADICT it! KNowing the Amalekite are cowards and only come forth when they think they can win MOses was bringing his hands up and down. This was also weeding out the imperfect Israel before we arrived at Mt Sinai where the three days of abstention brought everyone to a new "human" level. The imperfect fighters that survived Amalek could now become the Erev Rav adding a new internal problem greater than Amalek.
Logically, the Erev Rav will now get their chance to fight Amalek and die doing it, in this way they will finish their tikkun very honorably indeed. In my opinion Amalek is no match for these folks, but Gd is on the side of the one who is fleeing not the one who is chasing, so obviously this is going to be an epic battle.