1)
Salva is scared of the dangerous animals ( e.g. Lions ) because it ate Marial and they can attack again.
2)
I think that the desert is the worst because there is no shelter or trees anywhere. There is also no water anywhere just sand and a flat hot ground.
3)
He motivates him to keep walking so he doesn't die and he helps him. He gives him water so he can carry on. If you keep going you will succeed on it. And that moment when Salva was about to stop uncle said that this is the worst bit. And it will be easier soon.
4)
What I think the challenge that Salva has to overcome is when he wants to give them water but he can't.