OK, I'm going on my hazy memory of Japanese history here. Bascially GOJOE is a revisionist take on the Benkei/Yositsune (ie Shanao) story/myth which has been a staple tale in kabuki theatre for hundreds of years. The Gojoe incident is a small part of the story, after shanaos defeat he teams up with benkei and they go off to shanaos homeland, fight wars, have adventures, etc.
The film GOJOE is an attempt to subvert such a myth, saying that both died in the final fight on the bridge, and were replaced by shanaos companions (one of whom is a girl btw)- who i think were his retainers/vassals.
Having Masatoshi Nagases character go blind at the end is keeping with this new take on the story, since he is the only survivor who knows the truth about the fight on the bridge he couldn't point out the imposters even if he wanted too.
hope that makes sense!
The little companion of shanao might have supposed to have been a girl, but the actor who play the character, as austin power would say, was a man, baby!
I will say on retrospect he looked rather girlish though.
I would have to say that the younger one was his younger brother and took his name and the head of the clan after his death, he was the kagemusha, or the "stand-in" for the leader if he was ever to die, or was not present for whatever reason. Since no one knew what Shanao looked like, since he was in hiding most his life, then the younger brother was able to just take his identity.
I'm pretty sure the older guy was his bodyguard, probably apointed long before the story took place.
"A government is like fire, a handy servent but a dangerous master." -Geroge Washington
he couldn't have been the younger brother because in the flashbacks and in the present they talk of the prince being the last of his bloodline.