On what grounds did Yehudah request to switch with Binyamin - as Yosef's slave?
Rashi: Because, Yehudah pointed out, he was superior to Binyamin in all areas - in physical strength, in warfare and to serve. 1
Seforno: In order to avoid sinning to his father all his days. 2
Malbim #1 (to 44:19, citing Mahari): If the king declares a monetary punishment or slavery on someone, he can switch it (to accept payment from someone else); this is within his power.
Malbim #2: In any case I cannot return home without Binyamin, lest I see the evil that will befall my father (44:34).
If Yehudah remains in place of Binyamin, will he fulfill "v'Hitzagtiv Lefanecha (I will stand Binyamin in front of you)"?
Ohr ha'Chayim: Yes - his brothers will be Sheluchim to do so; it is as if Yehudah does so.
