From what I got Radcliffe character made him feel like -he- wanted to walk out and force the glasses wearing character to take the raft with the other man who for most of the walk in the jungle was belittling him and mocking him. The glasses wearing man decided the best course of action for him would be going with the hunter who was “papa” as he nicknamed him even though at the end of the movie we find out that did not end well for him
Yossi wanted to make sure that Marcus would be able to make it back to La Paz. While he initially wanted to take the raft, he offered to finish the hike with Karl so Marcus wouldn’t need to walk. Marcus revealed that he had been taking care of his feet like Karl taught him and so his feet had become much better.
Realizing that Marcus could walk, and that Kevin was acting hostile to both Karl and Marcus, Yossi agreed to let Marcus hike with Karl while he, Yossi, would take the raft with Kevin.
Could the four of them just hiked the rest of the way? Of course they could have. However, Kevin was not going to continue hiking with Karl and Karl was not going back into the river.
I didn’t hear or see any kind of subtext, plan or plot by Yossi to manipulate or force anyone (as the other posters indicated). Yossi was probably the most sincere character of the entire group. In my opinion, he just wanted the best outcome for everyone.
The problem with any explanation other than a bad script (or stupid people if this is a true story) is that they've been hiking for days trying to get out of the jungle and everyone is talking about the rainy season coming. At that point, intelligent folks put aside their differences and walk the three days together until they can safely go their own ways. The movie is well made but I don't think the characters follow the action of any reasonable person.