Nowhere do we find that the angel returned, so how could he say "Shov Ashuv"?
Rashi #1 and the Ibn Ezra: The angel was speaking (not in his own name, as how could he promise him a child? but) in the name of Hashem, 1 who returned, despite the fact that the Pasuk does not mention it.
Rashi #2 (in Seifer ha'Pardeis): He was hinting (not to the initial birth of Yitzchak, but) at his rebirth - at the Akeidah, where the same angel called out to Avraham to stay his hand.
Ramban: The angel was referring, neither to Hashem nor to himself, but) to the turn of the year (when the same time next year comes round. 2 , as He promised him earlier (in 17:21).
What are the ramifications of the fact that the Torah writes "ka'Eis Chayah" with a 'Kamatz'?
Rashi: It means 'You will be alive then (the following year) like you are alive now.' 1
Moshav Zekenim, Rosh: Had it written ka'Eis with a Sh'va, it would have meant on the due date of birth (nine months later).
Why did the Shunamis accuse Elisha of lying when (similar to the angels here), he promised her a baby at the same time next year?
Rashi (citing a Midrash Rabah): Because, unlike the angel, 1 Elisha did not promise that he would return the following year. To which Elisha replied, that unlike the angels, he was a mere mortal and there was no guarantee that he would be alive one year hence.
Why does the Pasuk suddenly switch from plural to singular (with reference to the angels)?
Rashi (on Pasuk 2) 1 : Because only one of the angels was sent to inform about the birth.
After informing us that Sarah heard the angel's words at the entrance of the tent, why does the Torah see fit to add that he was behind the angel?
Targum Yonasan: the word "Hu" refers (not to the entrance of the tent but) to Yishmael, who overheard the angel's prophecy.
Seforno: It was because the entrance to the tent was behind the angel who was speaking that he did not speak to Sarah directly as did Elisha 1 to the Shunamis.