Why does the Torah break off the narrative here, to discuss the lineage of Reuven, Shimon and Levi?
Rashi #1 and Rashbam #1: It is important at this juncture to present the Yichus of Levi on account of Moshe and Aharon. 1 And it would not have been befitting to begin with Levi, 2 so it begins with Reuven.
Rashi #2 (citing the Pesikta Gedolah) and Rashbam #2 (citing the Mechilta): The Torah makes a point of presenting the lineage of Reuven Shimon and Levi, to demonstrate their importance, since Ya'akov berated them before his death. 3
Seforno: Because all those listed, were appointed leaders of the generation of those who came down to Egypt 4
Oznayim la'Torah: In case one asks as to why Malchus and Kehunah were given to Moshe and Aharon respectively and not to someone from the tribe of Reuven, who was the firstborn or from Shimon, who was born second, the Torah presents the names of the members of all three tribes to demonstrate that there was nobody in Shimon and Levi who could compare to Moshe and Aharon
Ramban: Thereby conveying the impression that Levi had now become the firstborn, in honor of Moshe.
Rashbam: And it makes a point of mentioning Lorach, the sons of Uziel and Pinchas, so that, when the Torah discusses them later we should know who they are.
See Seforno, who explains why the Torah lists only the sons of Reuven (the B'chor) and Shimon, but also the grandsons of Levi
If, as the Midrash Rabah states Reuven remained the B'chor regarding inheritance, why did he not receive a double portion in Eretz Yisrael?
Oznayim la'Torah: Because the Midrash holds that Eretz Yisrael was 'Ra'uy' (they only inherited it after they captured it). Consequently, even though Reuven remained the B'chor, he did not receive a double portion because he inherited in Eiver ha'Yarden, before they had captured Eretz Cana'an.
Why does the Torah tell us how long Levi lived?