...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.
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---