Why does the Torah add the 'Vav' in "va'Hashem"?
Ramban #2 (citing Midrash) and Rashbam 4 : It refers to Gavriel, speaking in the name of Hashem.
Ramban #3 (citing Midrash), Targum Onkelos and Targum Yonasan 5 : It refers to Hashem, and the Torah often speaks like this.
See Sifsei Chachamim.
Riva citing Chizkuni: It says "va'Shem Shamayim Asah" (before angels were created)"! Rashi discusses when it says so about punishment. Or, Hashem joined Midas ha'Rachamim to Midas ha'Din, like Rashi said. It says "va'Shem Pakad Es Sarah"! Because Pharaoh took her and she was not defiled, the Beis Din ruled that it is proper that she give birth - "v'Im Lo Nitme'ah..." It says about Makas Bechoros "va'Shem", and it was "I, and not an angel"! This means that Hashem would not rely on an angel alone. Rashi said that also Makas Bechoros was with His Beis Din. Hadar Zekenim says that the Hashem and his Beis Din struck them; Hashem alone passed through Egypt.
This suggests that the decision to destroy Sedom was unanimous, and that not a single member of Hashem's Heavenly Court had anything to say in their defense.
Targum Yonasan adds 'the word of Hashem'
To whom does "me'Eis Hashem" refer?
Rashi: It refers to Hashem, as if the Torah had written "me'Ito" (from Him). 1
Rashbam and Ramban (citing the Midrash): It refers to the Shechinah Itself.
Why does the Torah write "va'Hashem Himtir al S'dom ... " Which implies that He sent rain?
Rashi and Targum Yonasan: Hashem in His mercy, first sent down rain of blessing, to give the people a chance to do Teshuvah; and it is only when they didn't, 1 that the rain turned into sulfur and fire. 2
"Himtir" implies from the sky. Why does it mention "Min ha'Shamayim"?
Hadar Zekenim: We learn that Hashem punishes Resha'im from Shamayim, and bestows good to Tzadikim from Shamayim (e.g. Manna). A parable for Shamayim is an oven. One punishes his enemy from a hot spit from the oven, and heeds to his beloved hot bread from the oven