Why does Gene return to Devon after fifteen years in A Separate Peace, and what does this suggest about the novel's plot and structure?

Generally, I agree with "mshurn"'s point, but I would add that the return to Devon in the novel adds an essential dialectical element in the novel between the past and the future.  The primary story of the novel is set on the backdrop of WWII, which, for the adolescent protagonists, orients the world toward the future:

"When you are sixteen, adults are slightly impressed and almost intimidated by you.  This is a puzzle, finally solved by the realization that they foresee your military future"

The protagonists--Gene, Finny, Leper--also can foresee their military future (conveyed in subtle hints, such as the summer session, or the one-month September vacation which serve to expediate the graduation process).  Most of them respond to this with qualified excitement (even Leper is capable of finding "a recognizable and friendly face to the war").

However, by communicating this through the more nuanced vision of the older Gene Forester, Knowles gives the readers a point of view which is independent of any youthful idealism.  It is not a voice that condemns the horrors of war either, because, as Gene notes in the closing paragraphs, he never saw any action in the war and probably spent most of the time in Florida, but it does, nonetheless, communicate that the future which they had envisioned for themselves was actually much more real than they had hoped: the visions of the future, in the novel, depict the way that things should have been, the past the way that they were.