Why does the Pasuk suddenly switch to the singular?
Rashi: Refer to 18:2:1:5:1.
Targum Yonasan: Because after depositing Lot outside the own, Gavriel returned to S'dom to destroy it.
Why was Lot forbidden to turn round and look back?
Rashi: Having sinned alongside the Sedomians, Lot did not deserve to be saved on his own merit, only on the merits of Avraham. Consequently, he was not worthy to witness the death of his compatriots.
Ramban #1: The plague that struck Sdom was contagious, and like all contagious diseases, even looking at it and even thinking about it can be harmful. 1
Ramban #2: When Hashem destroyed S'dom, the destructive angel (whose very appearance was devastating) stood between Heaven and earth; therefore it was forbidden, precisely on account of the danger, to look at it. 2
Rashbam #1: In order to break with their sons-in-law who were still in S'dom.
Rashbam #2: So as not to slow down his escape.
Rashbam #3: In order not to see the angels at work unnecessarily. 3
Seforno: The plague that struck S'dom was contagious and would catch up with Lot should he delay. 4
Like for example, someone who has been bitten by a rabid dog and who looks at a cup of water, will see the image of the dog, go crazy and die. Indeed, that is why Lot's wife turned into a pillar of salt - because she saw the sulfur and salt that descended from heaven to destroy Sedom.
In similar vein, the Ramban cites the Pirkei de'Rebbi Eliezer, who says that the Shechinah descended to pour on Sedom and Amorah sulfur and fire. And it was when Iris saw the back of the Shechinah that she turned into a pillar of salt.
As indeed happened to Iris (his wife) when she disobeyed this order.
Why did the angel instruct Lot to escape to the mountains? What was special about the mountains?
Rashi: Because Avraham lived in the mountains, and once he arrived there, he would be safe. 1
How will we reoncile this Pasuk with the Pasuk in Lech L'cha 13:3, which records that Avraham's tents reached Chevron?