Why does the Torah refer to Reuven as the Bechor?
Rashi: Even as it mentions his son, it calls him the Bechor, to teach us that he did not lose his title. In fact, he remained the Bechor regarding inheritance, 1 the Avodah 2 and always being the first to be listed.
Seforno: Because he did Teshuvah, he did not lose his title b'Dinei Shamayim, 3 nor even b'Dinei Adam, until Yaakov removed it from him. 4
Ohr ha'Chayim: This is to teach that he did not sin. He was Bechor Yisrael - do not suspect him [of Zenus]!
And Yosef only received the Bechorah as regards being divided into two tribes. Hadar Zekenim, Moshav Zekenim - Reuven received a double portion of Yaakov's property.
Moshav Zekenim: Reuven served Yaakov two weeks [at a time], and the others, one week. Alternatively, before the Mishkan was erected, the Bechoros offered Korbanos.
Since Teshuvah (Refer to 35:22:5:1 and 35:22:6:3) reaches as far as the Heavenly Throne (Yoma 86a).
Because he was Chayav a Niduy (a minor form of excommunication) which only takes effect when it is pronounced by a Chacham (See Moed Katan 16a).
