To what do the three phrases 'Cursed is the one who bore her, cursed is the one who raised her, cursed is the one from whose loins she came' correspond?

Maharsha: The first phrase addresses her mother, the second phrase addresses her wetnurse and the third addresses her father.

Iyun Yaakov: The first corresponds to her childhood (until age 12); the second corresponds to her time of 'Naarus' (aged 12-12.5); and the third corresponds to her adulthood (aged 12.5) or marriage, when she left her father's domain.

Why did the 'threads of fire' enter specifically through the nostrils ('Chotmo') of Nadav and of Avihu rather than through the mouth, which is the way that death by burning enters (by placing into it a burning lead piece)?

Ben Yehoyada #1: Their nostrils sinned before their mouths, as their sin was unlawfully entering the Holy of Holies to burn Ketores. Physically, the nose enters a room slightly before the mouth.

Ben Yehoyada #2: They sinned by speaking against the two leaders of the nation, Moshe and Aharon. This corresponds to the Gemara in Taanis 29a, that refers to the great Rabbi of a generation as the 'Baal HaChotem'.

Benayahu: They sinned in unlawfully bringing Ketores - incense, which is smelled through the nostrils.

How could such great people as Nadav and Avihu speak this way about their father and uncle, the undisputed greatest Rabbis of Am Yisrael?

Chofetz Chaim (Biur on Toras Kohanim Mechilta D'Miluim 21): They were merely concerned that Moshe and Aharon would soon die and they would then need to lead the nation. However, they expressed it in an inappropriate way.

Minchas Ani: Hash-m had already signaled that the nation's greatest figures would die during the Mishkan's inauguration. As noted in Zevachim 115b, the Pasuk (Shemos 29:43),'and it shall be sanctified by My glory' is read as 'bimchubadai' ('by My honored ones'). Knowing that Hash-m is sanctified through the death of those closest to Him, Nadav and Avihu assumed the prophecy referred to Moshe and Aharon. Their comments were not a wish for their elders to die, but rather a concerned realization that the burden of leadership would soon fall upon their own shoulders.

Einei Shmuel: Nadav and Avihu understood that Moshe and Aharon were 'too holy' to lead the people into the Land of Israel. Chazal teach that had Moshe built the Beis HaMikdash, it could never have been destroyed; yet, the Mikdash's eventual destruction was a necessary 'safety valve' to spare the people from divine wrath. Therefore, they reasoned that because they were lesser in stature than Moshe and Aharon, they were actually better suited to lead the entry into the Land. Hash-m's response was that they were actually greater than Moshe and Aharon, and as a result, they too could not be the ones to lead the nation in.

Why was Avihu punished if only Nadav spoke this way?

Nachal Kedumim (Chida, citing Rabbanei Ashkenaz): There is a Midrash Pli'ah that teaches that when Iyov saw the death of Aharon's two sons, he exclaimed, "From this (too) does my heart quake" (Iyov 37:1). He realized then that he would also be punished for his past silence in the court of Pharaoh. (The Gemara in Sotah 11a teaches that Pharaoh consulted three advisors before enslaving Bnei Yisrael: Bilaam, who encouraged it; Yisro, who fled; and Iyov, who remained silent.) Similarly, Avihu's silence when Nadav spoke indicated his complicity; because he failed to protest his brother's words, he was held equally accountable.