Why does the Torah insert the word "Shemos"?
Seforno: Refer to 1:2:4:2.
What is the significance of the P'sik between the words "ha'Bechor" and "Nadav"?
Ba'al ha'Turim: To teach us that the word "Bechor" pertains to Aharon, who was Bechor 1 to Moshe, and not to Nadav who died without children - and whose Bechorah was therefore meaningless.
Etz Yosef (on Bamidbar Rabah 6:2): He was older than Moshe, but not the Bechor. Shmos Rabah (1:13) says that Miryam was two years older than Aharon. ("Va'Teled l'Amram Es Aharon v'Es Moshe v'Es Miriam" (26:59) implies that Aharon was older. However, it seems that they were not born in this order. Did Moshe have another sister who brought Yocheved to Bisya (Shemos 2:7,8), and the Torah does not mention her again?! - PF)