The interesting thing in Mark is that other than Mark v1:9 it is not clear that Nazareth is a town.
However, Nazareth in Mark doesn't appear to be a late addition. Nazareth appears to be part of the initial layer of the writing. There is reason to think that "of Galilee" in v1:9 may be a later addition by an editor, because it is not paralleled in the ending of Mark, "6 But he said to them, “Do not be alarmed; you are looking for Jesus of Nazareth, who was crucified."
Note the parallels:
Ending:
5 As they entered the tomb, they saw a young man dressed in a white robe sitting on the right side, and they were alarmed. 6 But he said to them, “Do not be alarmed; you are looking for Jesus of Nazareth, who was crucified. He has risen; he is not here. Look, there is the place they laid him. 7 But go, tell his disciples and Peter that he is going ahead of you to Galilee; there you will see him, just as he told you.” 8 So they went out and fled from the tomb, for terror and amazement had seized them, and they said nothing to anyone, for they were afraid.
Beginning:
6 Now John was clothed with camel’s hair, with a leather belt around his waist, and he ate locusts and wild honey. 7 He proclaimed, “The one who is more powerful than I is coming after me; I am not worthy to stoop down and untie the strap of his sandals. 8 I have baptized you with water, but he will baptize you with the Holy Spirit.”
9 In those days Jesus came from Nazareth of Galilee and was baptized by John in the Jordan. 10 And just as he was coming up out of the water, he saw the heavens torn apart and the Spirit descending like a dove upon him. 11 And a voice came from the heavens, “You are my Son, the Beloved; with you I am well pleased.”
12 And the Spirit immediately drove him out into the wilderness. 13 He was in the wilderness forty days, tested by Satan, and he was with the wild beasts, and the angels waited on him.
14 Now after John was arrested, Jesus came to Galilee proclaiming the good news of God 15 and saying, “The time is fulfilled, and the kingdom of God has come near; repent, and believe in the good news.”
The many parallels between the beginning and ending suggest that the beginning and ending are intended to mirror one another. Yet, the ending only has the one mention of Galilee, while the beginning has two.
This, along with the fact that Mark v1:14 reads as if Jesus is coming to Galilee for the first time, suggests that Mark v1:9 originally read just, "In those days Jesus came from Nazareth", not "Nazareth of Galilee".
So again, this begs the question, if Nazareth wasn't "Nazareth of Galilee", then what was it?