Why did Avraham opt to attack them at night-time?
Seforno: This was a clever strategy, to trick the kings into believing that there were more soldiers than there really were. 1
What does the Torah mean when it writes that "the night split up on them"?
Rashi #1: It was not the night that split up, but Avraham's army, in order to pursue the fleeing segments of the enemy, in the way that armies tend to do.
Rashi #2 and Targum Yonasan: It was the night itself that split up; 1 the first half for the current miracles, while the second half was reserved for the miracle of Makas Bechoros in Egypt.
Ramban: Avraham chased after them by day as far as Dan; then, at night, when he did not know which way they would flee, he split up his army into two or three groups to pursue them following all the possible routes that they might take.
Seforno: Avraham split his army into two, leading the enemies to believe that they were being attacked by the surrounding locals. 2
How could Avraham have pursued the kings up to "Chovah," when no such place exists?
Rashi and Targum Yonasan: "Chovah" is a synonym for "sin," and it refers to the image that was later set up in Dan (refer to 14:14:3:1*).
The verse says, "He struck them, and chased them." This seems to be out of order
Da'as Zekenim, citing Midrash Tanchuma: Avraham chased, and Hashem struck.