What is the significance of Hagar's calling out the Name of Hashem?
Rashi: She acknowledged that Hashem takes the side of those who are put to shame, as He took her side.
Targum Yonasan: She thanked Hashem for having answered her prayers.
Seforno: Calling out the Name of Hashem is an expression of Tefilah - since praising Hashem always precedes prayers 1 .
As the Gemara states in Berachos (32a).
Why did she refer to Hashem as "Kel Ro'i"?
Rashi: Because Hashem sees, i.e. He takes the part of those who are shamed.
Seforno: Because Hashem is the G-d of vision - He sees everything 1 wherever it may occur, even not in Avraham's house.
Targum Yonasan: Hashem sees but cannot be seen ...
Which is also how Targum Onkelus translates it.
Why was Hagar so surprised to see angels?
Rashi: Hagar was accustomed to seeing angels in the house of Avraham. 1 However, it is one thing to see angels in Avraham's home, and quite another, to meet them in the desert; and that was why she was so surprised.
Targum Yonasan: She was not surprised; what she was saying 2 was although Hashem cannot be seen, His Shechinah appeared to her (via an angel), after having angels earlier.
What does it mean "Acharei Ro'i"?
Riva: When the angel turned around, she did not see anything. She deduced that he was an angel.
רש"י: הגם הלום- לשון תימה: מהו "א-ל רואי" לפי זה? וכיצד משמע "רואה בעלבון של עלובין"?
גור אריה: פרוש הפסוק- אתה רואה בעלבון עלובים כי לא רק בבית אברהם יש מלאכים בשביל צדקתו, אלא גם כאן במדבר.