...so the Prince of Egypt tells our ten intrepid travelers that they dare not return to the land of the River g-d without their youngest brother firmly in tow. Okay, that's a problem because Father does not let said brother out of his sight. Great tension ensues.I'm going to try my hand at it, by asking another question, and one which had been asked before. How did Yosef even know about Binyamin? Well, Yaakov asked this:
The question is...why? Why such worry? Why all the grand statements by everyone involved? What's the big deal - simply grab some kid, throw a pair of tzitzis and a yarmulka on him and pass him off as the younger brother. How hard could it have been to pull off such a ruse? They don't know that it's Yosef back in Mitzrayim. As far as they know it's a crazy ruler who for some reason is picking on them.
So why not pass off another as Binyomin, it would seem to be a no brainer way to solve all the problems at hand.
---Yes, there is a clear answer---
In other words, Yosef was grilling them. Indeed, within parashat Vayigash, there are extra details of this which were unmentioned previously. Take it from their perspective. They were accused of being spies, and to prove this not so, produce this brother. Wouldn't Yosef grill Binyamin, to see if the story matches? It is no mere matter of saying "I am Binyamin". He would also need to know details of family history, and these would need to match precisely those put forth by the brothers. It would be dangerous to transform themselves from truth-tellers into liars.
Besides, they kept the entire Torah, which states midevar sheker tirchak.