Why did Yehudah lie by stating that Binyamin's full brother was dead?
Rashi: He was afraid that if he intimated that he was still alive, Yosef would demand that they bring him down to Egypt. 1
Chizkuni asks, they had already said "veha'Echad Einenu" (42:13) - so Yehudah now contradicts himself! Riva - That term can mean that he died, as in the Pasuk, "v'Einenu, Ki Lakach Oso Elokim" (Bereishis 5:24). (This is unlike the Midrash that they had already told Yosef that they had come to search for him; refer to 42:13:1:1 - EC). Also see Mizrachi and Gur Aryeh (44:20:1.1).
Why did Yehudah give all of these details?
Malbim: All of them show that Binyamin is pampered and weak. (a) His father is old, and longs for him excessively. (b) He was born in his old age, so he pampers him more than his other sons. (c) Since Binyamin is small, 1 his father longs for him more. (d) Binyamin's [maternal] brother died, so his father has more mercy on him. (e) He is the last son left from his mother. (f) His father loves him for his own sake (attributes).
Ha'amek Davar: He longs for him like for a small son (even though he is 30).
Why did Yehudah say, "va'Yivaser Hu Levado l'Imo"? Yaakov had used a different term, "v'Hu Levado Nish'ar" (42:38)!
Ha'amek Davar: There is a difference between Nish'ar and Nosar. Nish'ar is when one intentionally leaves over. Nosar implies automatically. Yaakov said that through Hashgachah, Hashem left only Binyamin to Yaakov, to fulfill "Goy u'Kehal Goyim Yihyeh Mimeka" (35:11), when Yaakov intended to give the Bechorah to Yosef; now it will be from Binyamin. In any case, he was worried lest fatality occur to him, lest sin cause (Bitul of the promise). Further, Binyamin already had sons, so there is no guarantee that he not die. The brothers 1 did not know all this, so they said that he is Nosar Levado l'Imo.
Even though now only Yehudah spoke, he says that he already told Yosef so, even though the Torah did not record it above. (PF)
Why did he say "Aviv," and not 'Avinu'?
Ha'amek Davar: He longs for him as if he is a lone child, and he is the father only of this son. Since they said [that he loves him so much] before Yosef asked them to bring him, it is clear that it is true; it is not an excuse not to bring him.
QUESTIONS ON RASHI
Rashi writes: "His brother is dead' - Out of fear, Yehudah let a falsehood escape his mouth." But earlier, they had said, "... and one is missing (Einenu)" (42:13); Rashi (loc. cit.) explains that they had told the viceroy, "We [entered from ten different gates] in order to spread out and search for him." Wouldn't this obvious changing of their story land them in even greater trouble?
Mizrachi: Already when they arrived before Yosef the second time, bearing their gift, Yosef inquired about the "missing brother;" and they responded that they had heard that he died. 1
Gur Aryeh: Yehudah now explained what they had meant by "Einenu" (42:13). 'We presumed that he had died (for he should have returned by now); we searched for him only because we did not know this for certain.' (This was false; in fact they did presume that he was still alive.)
Gur Aryeh: How would Yosef have asked such a question?! He was Yosef; what did he expect them to respond?!
Rashi writes: "'His brother is dead' - ... If I say that he is still alive, he will tell me to bring him!'" But even if this was merely a re-interpretation of their story (and not an outright change - refer to 44:20:1.1:2), it would likely arouse suspicion, leading the viceroy to demand that he appear!
Gur Aryeh: Yehudah had to emphasize now that Yosef was presumed never to return; in order to explain Yaakov's deep attachment to the one remaining brother, Binyamin. 1
Gur Aryeh: If this was Yehudah's intent, why does Rashi write another explanation? It would have sufficed for Yehudah to say that, 'our father fears that his brother will never be found' - which would be true! Out of fear that he would still have to produce Yosef before the viceroy, he did not say this.
Rashi writes: "'[He] alone [remains] to his mother' - i.e. he has no other brother from that mother." What is Rashi explaining?
Gur Aryeh: We should not explain as 'his mother has no other son;' for in fact his mother (Rachel) was no longer alive. Rather, it means that Yaakov has no other remaining son from Rachel.
