Why did Moshe call his first son Gershom, and then his second Eliezer, even though he was saved from the sword of Pharaoh before he became a stranger in a foreign land?
Seforno: Because it was only after the current king of Egypt died that Moshe was safe from his sword - and that was only after Eliezer was born. 1
Rosh (to Shemos 18:4): Explains in a similar vein; that Moshe was not completely safe from the sword of Pharaoh until those who were after his blood had died (See Rashi to 4:19).
What is the significance of the fact that Moshe called his son "Gershom," and not 'Gershon' - like Levi did?
Beis Yosef, Even ha'Ezer, 129: This teaches us that Gershom and Gershon are two distinct names and that one may not confuse them in a Get. 1
See Torah Temimah, note 45, who elaborates.
Why does it not say that Tziporah became pregnant?
Moshav Zekenim: She was young and modest, so her pregnancy was not discernable. 1
This is how Chazal learned regarding Zilpah (although the Midrash regarding Zilpah does not mention modesty). Refer to Bereishis 30:10:1:1.
