The phrase on the edge can be used in many ways with many different meanings. It can mean being on the edge of a cliff or rock face or it can mean being close to making a final decision. On edge can also mean someone is very tense, and also being close to something happening, generally bad or disastrous. I think that it can refer to any of these as it is used in the title.
Foreshadowing is a language feature that is used by Joe to hint that something bad might happen. An example is that Joe often talks about how high up on the cliff they are and also how dangerous it would be to fall and would most certainly end in his and Simon’s death. This makes the reader feel that Joe and Simon might experience a fall similar to one Joe describes that Simon has experienced. Simon was climbing and he watched two Japanese climbers fall to their death, I feel that this hints towards the fact that Simon and Joe might also fall down and suffer the same fate