How were the two Pillars placed?
Rashi, Rashbam and Targum Onkelos: When the evening arrived, the Pillar of Fire came down in front of Yisrael as usual, whilst the Pillar of Cloud, instead of leaving, moved to the back of Yisrael, to make it dark for the Egyptians, and to intercept the arrows and slingshot that the Egyptians were firing at Yisrael. 1
Targum Yonasan: The Pillar of Cloud moved to the back of Yisrael, where it became half Cloud - to make it dark for the Egyptians) and half fire - to light up for Yisrael.
This can be compared to a father walking along the road, his son walking in front of him, when robbers attacked them and tried to take the son captive. What did the father do? He picked up his son and placed him behind him. When a wolf approached from behind, he picked him up and placed him on his shoulders. And when subsequently, robbers attacked from the front and wolves from behind, he placed him on his shoulders and fought them off, as the Pasuk hints in Hoshe'a (11:3 Rashi citing the Mechilta).
This was necessary in order to enable the Egyptians, who were able, through the Pillar of Cloud, to see the Camp of Yisrael moving forward and follow them into the sea (Ramban).
Both of which were led by the angel (Seforno).
If the Pillar of Cloud made it dark, what does the Pasuk mean when it adds that it lit up the night?
Rashi #1, Rashbam and and Targum Onkelos 1 : It means that the former made it dark for the Egyptians and the Pillar of Fire (referred to earlier in the Pasuk) lit up the night for Yisrael.
What is the Torah referring to when it writes "ve'Lo Karav Zeh el Zeh (in the singular) Kol ha'Laylah"?
Rashi and Targum Yonasan: It is referring to the two camps (of Yisrael and of Egypt). 1
According to the Midrash it refers to the angels, about whom it is written "ve'Kara Zeh el Zeh" (exchanging the 'Beis' of "Karav" for an 'Alef'), meaning that the angels did not sing Shirah that night - because the creations of Hashem (the Egyptians) were being destroyed.