What is the significance of the word "Vayashkifu" (as opposed to "Vayir'u" or "Vayabit"?
Seforno: Because they were the antithesis of Avraham, who was the very embodiment of Chesed - as the Pasuk in Yechezkel (16:49), describes S'dom (among their other detriments) as "refusing to support the poor and the destitute".
Since when do angels need to be accompanied?
Rashi and Seforno: Avraham still believed them to be men. 1
What is the significance of the phrase "ve'Avraham Holech Imam Le'Shalcham"??
Seforno: It was because Avraham was performing the Mitzvah of accompanying his guests that Hashem informed him of His intentions to destroy S'dom, so that he could perform another Mitzvah (of Chesed). 1
Moshav Zekenim: Because Avraham himself accompanied his guests, Hashem Himself led Bnei Yisrael in the Midbar.
In keeping with the Mishnah in Pirkei Avos (4:2) 'One Mitzvah leads to another. It also demonstrates the difference between Avraham (who typically was busy with an act of Chesed, whilst S'dom was the very antithesis of Chesed. Refer to 18:16:1:1a. Moreover, it highlights the greatness of Avraham Avinu, who Davened for S'dom despite this.
Which angels departed?
Rashbam and Targum Yonasan: The two 'junior' angels (Refa'el and Gavriel) now left for S'dom, whilst the senior of the three (Micha'el) was still talking to Avraham. 1
Presumably, he is the angel conveying Hashem's words to Avraham up to Shelishi. According to Targum Yonasan, he then returned to Heaven.