Why were Bnei Reuven taught here?
Radak: Reuven and Shimon were neighbors, so Reuven is taught after Shimon.
Why is he called "Bechor Yisrael"?
Malbim: He was the first Bechor born (to Yakov; others were Bechoros to their mothers).
Why does it say "Hu ha'Bechor uv'Chalelo Yetzu'ei Aviv Nitenah Bechoraso l'Yosef"?
'Rashi': It is lest one ask, since he is the Bechor, why did kingship not come from him?
Radak: He is the Bechor, and it would have been proper to begin the tale of lineage with him, for he should have had also kingship. His father had a cherished son, so when Reuven profaned his father's bed, his Bechorah was given to Yosef.
Malbim (2, b'Derech Drush): The Shevatim were unsure whether their status was like Bnei Noach, or like Yisrael. If they were like Yisrael, Bechorah belongs to Yosef, for Yakov was Mekadesh Rachel via Kesef (his Avodah for Lavan), before he was Mekadesh Leah 1 . The marriage to Leah was improper (she was Achos Ishto). If they were Bnei Noach, who have no Kidushei Kesef, Leah's Kidushin was first, and Bechorah belongs to Reuven. Reuven profaned his father's bed - this could be fixed only according to Din Yisrael, for one may marry a woman that his father raped or enticed, or also his Pilegesh, who has no Kidushin. Yakov took the Bechorah from Reuven to teach that there was no Kidushin [with Bilhah], so he did not sin with Eshes Aviv.
Lavan was a Goy, who cannot be Mekadesh his daughter! Perhaps Rachel (who converted) agreed to become Mekudeshes via Yakov's work for Lavan; it is considered as if she received it (Din Arev - Kidushin 7a). Still, how does this affect Bechorah? Even a Mamzer Bechor receives a double portion (Rambam Hilchos Nachalos 2:13)! (PF)
Why does it say "v'Lo Lehisyaches la'Bechorah"?
'Rashi': If one will ask, since Yosef became the Bechor, why did kingship not come from him? Even had Reuven not lost Bechorah, Yehudah was proper for kingship.
Radak: Yosef did not become the Bechor totally. If so, he would have received kingship, just like Reuven should have had also kingship.
Malbim: Only the right to inherit double was given to Yosef - two Shevatim came from him. For lineage, Reuven remained the Bechor, for he was born first. One opinion in the Midrash says so; the other says that Yosef became Bechor also for lineage.
