Why does the Torah see fit to insert the first phrase in the Pasuk?
Targum Yonasan: To tell us that they were all Tzadikim. 1
Even Reuven, Shimon and Levi, whom Ya'akov had rebuked.
How can the Torah write that Ya'akov blessed the twelve tribes, seeing as he rebuked Reuven, Shimon and Levi (and did not bless them)?
Rashi: The additional phrase "and he blessed them" teaches us that all twelve sons (including Reuven, Shimon and Levi) were included in the blessing. 1
Seforno: "and he blessed them" means that he gave them all a Berachah over and above the Berachos listed above.
What is the significance of the phrase "v'Zos Asher Diber Lahem Avihem"?
Hadar Zekenim, Rosh, Ba'al ha'Turim: Ya'akov ended his Berachah with the word v'Zos, and that is the word with which Moshe began blessing Bnei Yisrael (v'Zos ha'Berachah). 1
Taking his cue from Ya'akov (as David ha'Melech wrote in Tehilim (119:100) "mi'Zekenim Esbonan". Hadar Zekenim - Yitzchak concluded his Brachos to Yakov with "va'Yikra Yitzchak El Yakov", and Yakov began with "va'Yikra Yakov." Moshe ended with "Ashrecha Yisrael", and David began [Tehilim] with :Ashrei ha'Ish."
What does the Torah mean when it writes "Ish Asher k'Virchaso Berach Osam"?
Rashi and Rashbam: It means that each one received the Berachah that would later apply to him. 1
Seforno: He gave each son the Berachah that suited him personally - Yehudah, concerning Malchus; Yisachar, concerning Torah and Levi, the Avodah.
As Ya'akov specifically stated in Pasuk 1 (Rashbam).
Why, following "Ish ke'Virchaso" (singular) does it write "Beirach Osam" (plural) and not 'Beirach Oso'?
Rashi: To teach us that, although Ya'akov gave Yehudah the strength of a lion, Binyamin, the 'Chatifus' of a wolf and Naftali, the agility of a gazelle, he incorporated each tribe (including Reuven, Shimon and Levi) in the Berachos of each of the other tribes.
רש"י: יכול שלא כללן בכל הברכות וכו': וכי היו כולם שווים בברכתם?
גור אריה: כוונת רש"י שביהודה יש גבורת ארי לגמרי וגם לראובן יש קצת דמיון לארי, ויעקב כלל את כולם שבדבר מה כולם שווים בברכות.