Why does it mention that Menasheh and Machir were the Bechoros?
Rashi: This is why he was first to receive, in the days of Moshe, in Ever ha'Yarden.
Radak: He was the Bechor of Yosef's Bechor. Due to his strength, he (his sons - Bamidbar 32:39) conquered Gil'ad. Moshe hinted to this - "Bechor Shoro? Amim Yenagach" (Devarim 33:17) 1 . Bnei Gad and Bnei Reuven received in Ever ha'Yarden due to their flocks.
Bi'ur ha'Gra: The lottery was in the name of Menasheh 2 , for he was Yosef's Bechor.
Malbim: This is why Menasheh was divided. He was the Bechor of Yosef, and esteemed, mighty and populous, and his Bechor was a warrior. Yosef was torn (received partially on each side of the Yarden) because he caused the others to tear [their clothes when the cup was found with Binyamin]. Menasheh was torn (and not Efrayim), for he was the Bechor. 3
The verse attributes more [Gevurah] to Efrayim than to Menasheh - "Rivevos Efrayim? Alfei Menasheh"! (PF)
Bi'ur ha'Gra (14): Efrayim and Menasheh together received one portion, just it was split into two parts. Refer to 17:14:1:2*.
This is like Lekach Tov (Bereishis 37:34), that Menasheh was torn due to Yosef. Perhaps Menasheh was not the Shali'ach who caused the Shevatim to tear. Alternatively, Yosef told him that there is a Heter and need to accuse them falsely, just like he needed to call them spies and incarcerate them. Menasheh was obligated to fulfill his father's command, just like Yakov obeyed Rivkah. In such cases Yesh Shali'ach l'Devar Aveirah (Bava Metzi'a 10b), so only Yosef is punished for this. Or, Malbim means that since he was the Bechor, presumably, he was the Shali'ach who caused the Shevatim to tear. Lekach Tov Matos 140b-141a and Bereishis Rabah 84:20 say that Menasheh was torn because he caused the Shevatim to tear. - PF)
Why does it say that Mechir was Gil'ad's father?
Bi'ur ha'Gra: Mechir took Bashan, and his son Gil'ad took Gil'ad.
