Question: Greenwall says that the warrior must stand on the tower for five days. Wouldn't he die of dehydration?
Answer: As someone else said, somebody could give him water. The test might be more about staying in one place and denying himself the daily activities and pleasures of life - not having access to his usual amount of water and food.
Question: Does anybody know who threw the darts at Ace? It couldn't have been Gahjii or Katie because they're both wearing shoes and the thrower was barefooted.
Answer: Since we don't see the attackers except for their feet, it could easily have been Katie or Gahjii. In fact, given the speed at which the final four darts were shot into Ace, it was likely both of them. They could easily have not been wearing shoes at that point.
Answer: Not if somebody climbed up and gave him some water to drink.