Why does the author leave the narrative at Devon at this point and discuss the war as Gene later experienced it?

Remember that your are only allowed to ask ONE question -
five seems to be taking it somewhat to an extreme, so I will focus on your first
question, which refers to the last page of the novel where the action shifts from the
younger Gene to the older Gene looking back at himself and reflecting on what he learned
from the experience. 

To answer this question you need to
focus on the specific narrative style that is used in the story. It is called first
person retrospective narration, which means it is an older narrator looking back at his
or her younger self. This is a very specialised form of narration used in other novels
such as Great Expectations and Jane Eyre. What
is interesting about it is that we as readers need to remember that alongside the
youthful narrator explaining what is happening to him or her, we have the older, maturer
narrator who also comments on the action, sometimes with
disapproval.

What is fitting about this narrative departure
at the end of this novel is that it reflects what Gene has learned from his experience
and re-states the theme of the novel. Note how the older Gene does not share in the
other boys' disillusionment, because he is able to reflect more deeply on what has
happened. The older Gene reflects that the war is something far deeper and intrinsically
a part of the human condition. The older Gene suggests that war emerges from a kind of
ignorance deep within the human heart. It is this condition of ignorance that causes
people to seek out an enemy and envision the world as a dangerous, hostile environment.
The novel ends with the older Gene questioning the logic and worth of such an
approach:

All
of them, all except Phineas, constructed at infinite cost to themselves these Maginot
Lines against this enemy they thought they saw across the frontier, this enemy who never
attacked that way - if he ever attacked at all; if he was indeed the
enemy.

Thus the end of the
novel marks a narrative departure to give us this maturer ruminations - which would be
unrealistic for the younger Gene to present.