From where exactly did the angels depart?
Seeing as it wasn't Avraham who came there to speak to Hashem, but Hashem who came to speak to Avraham, (See Pasuk 20), the Torah ought to have said that Hashem was still standing over Avraham!
What is the significance of Avraham standing before Hashem?
Rashi: Although the angels had already gone to S'dom, Hashem was still 'standing' before Avraham, 1 waiting for him to Daven on their behalf.
Seforno: Even though the angels had already arrived in Sedom to destroy it, Avraham stood before Hashem Davening for mercy and to defend them. 2
Rashbam: Avraham was standing before the angel (as the Shali'ach of Hashem) to beseech him (on behalf of Sedom).
Targum Yonasan: He was Davening on behalf of Lot. 3
How, does this Pasuk fit chronologically into the Parshah?
Ramban #1 (citing the Ibn Ezra): The Torah writes it to inform us that it was after the angels arrived in S'dom, that Hashem said to Avraham "The cries of S'dom and Amorah are great" (Pasuk 20).
Ramban #2: Seeing as Pasuk 20 was said to Avraham, it makes more sense 1 to say that after the angels left for S'dom, Hashem said Pasuk 20, following which Avraham 'was still standing before Hashem, pleading with Him to have mercy on S'dom and to ask permission to speak. Then, after they arrived in S'dom (the current Pasuk), he Davened the Tefilah that the Torah now records. 2 Alternatively, "Avraham was still standing before Hashem", and he continued his Tefilos with Pasuk 23.
Since, according to the Ibn Ezra (Refer to 18:22:4:1), it is not clear why the Torah did not insert this Pasuk before Pasuk 20.
According to the Ramban, the question remains, the Torah ought to have written here that 'the men arrived in S'dom'. "The men departed from there", when all's said and done, belongs before Pasuk 20?