Why is there so much Death by Adaptation in the film? Because this is the only way to give the dead a voice. Almost none of those who died managed to relay their last thoughts and feelings before their deaths, compared to the many many accounts of those who survived. Yet it are (obviously) the dead who suffered most and whose fates, including what they felt and did before their deaths, are most important. So the stories of survivors (many of whom survived by sheer luck - the bullet missed them by inches of the wound turned non-fatal) are given to those who were less fortunate.
This is a general reason why Dramatization makes for very powerful films - the viewer knows this is close to how it played out in reality, but due to adaptational changes, neither Doomed by Canon nor Saved by Canon apply - unlike in documentaries, you cannot just Google the fate of your favorite character.
Given the number of encounters Kaja has with the terrorist, one can think this is massive overdramatization. However, if one compares Breivik's route here with Kaja's route (club house - forest to the west - tents - again forest to the west - beach southeast - rocks southwest - again beach southeast), one sees that all encounters are plausible. The only exception is the final shot that killed Kaja, which was freely invented, rather than taken from survivors' accounts.
Why does the film overplay the bleakness of the situation by excluding all rescue attempts (until the very end) and making swimming to the mainland seem nearly impossible? Because that's how those hiding in the rocks on the Love Path felt. They did not consider possible to swim directly to mainland, or they wouldn't hide in the rocks (those who did consider it possible had already swam away by the time Kaja arrives on the beach). The boats weren't there initially. By the time the evacuation started, Breivik appeared on the Love Path and made it impossible for the boats to come closer than 150-200 m to the shore, which for most of those hiding was still unreachable, not to mention that by trying to swim to the boats, they would be in the line of fire. Many of those hiding in the rocks weren't evacuated until 30-60 minutes after Breivik was arrested. For them, the situation really was as bleak as shown.
Why did Kaja made such a stupid mistake near the end, standing completely in the open, and arguing/sobbing loudly? Because she moved past Despair Event Horizon and lost the will to live. This may also possibly explain why in the Berlinale version, she died so fast, and without any last words.
Remember the wounded girl? Her parents will never find out about her last moments, now that Kaja died before telling the story to anyone.
The wounded man in the boat in the end of the film is older, and he was in the boat when it stopped, not in the cave which implies that this is the boat's owner. This in turn means that the boat was shot at as it approached - and Emilie, Kaja's sister, was in the boat. It's just sheer luck that she was not killed as Kaja was.
The name "Kaja" is probably derived from the Norwegian word "kaia", meaning "dock". The Utoya dock was the place where first victims died. So it's fitting that Kaja is the last victim.