In the 9th chapter the one being held in the dude's arms is actually the real Theodore and the one supporting Arlo's side is the real Prince(Radol). And to the panel where we see a hand and Arlo (ch9), was the prince's hand letting go of Arlo and moving away from sight, that's when Arlo panicked.
THANK YOU FOR THE EXPLANATION （/TДT)/ you're a hero
The panel is a little messy but it was Arlo holding Theodore in his arms and in next scene we can see that Theodore jumped out of his arms and run away (most likely info fire). The Radol run away alone at the very beginning like they planed (most likely to hide away in the brothel as "Theo"). Arlo thought that Theodore died as "prince" as it was planned from the beginning. The ending is quite open because it might be Radol who came back or Theodore who miraculously somehow survived (but that is less likely). I think when both boys tried to run away for the first time and the one that was saved and brought back was the child who worked as helper in the brother but the guard who found him (and was also torturing Arlo in present time) said that it doesn't matter if the real prince died long time ago because they are going to make it look like "prince" died now in fire.
I think, the prince (Radol) and the substitute (Theodore) switched places and names before the real prince drowned (ch 7), the prince was drifted at the place where the substitute once worked at, thus why he now works at the brothel. And the substitute is acting in his stead. Acting like the villain to make the palace traitors (see ch7) THINK they manipulated the substitute into killing the real prince.
And also on ch 8, the real Theodore worked at the brothel in his childhood days, until he was hired to act as the prince's substitute. And he didn't want to go back there(the brothel) so he felt guilty staying in the palace ch8.