What is the significance of Hagar's calling out the Name of HaSh-m?
Rashi: She acknowledged that HaSh-m takes the side of those who are put to shame, as He took her side.
Targum Yonasan: She thanked HaSh-m for having answered her prayers.
Seforno: Calling out the Name of HaSh-m is an expression of Tefilah - since praising HaSh-m always precedes prayers. 1
As the Gemara states in Berachos 32a.
Why did she refer to HaSh-m as "Kel Ro'i"?
Rashi: Because HaSh-m sees, i.e. He takes the part of those who are shamed.
Seforno: Because HaSh-m is the G-d of vision - He sees everything 1 wherever it may occur, even not in Avraham's house.
Targum Yonasan: HaSh-m 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; that is why she was so surprised. 2
Targum Yonasan: She was not surprised. What she was saying 3 was although HaSh-m cannot be seen, His Shechinah appeared to her (via an angel), after having seen angels earlier.
As is evident from the fact that she displayed no fear upon seeing four angels, one after the other (as opposed to Mano'ach, Shimshon's father, who was afraid that he would die, after seeing only one - Shoftim 13:22).
Gur Aryeh: Rashi explains the verse as follows: "You, HaSh-m, see the insult of the downtrodden! Not only in the house of Avraham did I see angels [that appeared due to his merit], but even here in the desert; for You saw [my] insult."
What did Hagar mean by, "Acharei Ro'i"?
Riva: When the angel turned around, she did not see anything. She deduced that he was an angel.