[Asceai]

The routes that best explain the Eternal World are probably Akane's and Hikami Shun's, but it's mostly an interpretation thing. Like pretty much all Key games (I consider ONE and MOON. both Key games, even if they weren't actually made by Key =p) it's a somewhat 'magic realist' setting. Spoilers follow:

Spoiler:

Kouhei's loss made him desire an unchanging existence (where the ephemeral joys of childhood would never disappear). The 'pact' with Nagamori was to leave this world and exist only in the Eternal World. This transition seems to be completely impossible to stop. However, if Kouhei made enough of a mark so as to remain in the real world in some form, he would be able to return after a year. That's why the only person who actually remembers Kouhei during this time is the chosen girl (or Hikami Shun in hisending). Of course, after Kouhei's return, everyone else remembers him too.

Because of that, the fact that Akane's childhood friend never returned - despite Akane thinking about him virtually all of the time - seemed a bit odd to me. Perhaps her feelings for that childhood friend weren't strong enough (like what happens to you if you screw up near the end of a girl's route).

As for it not being integrated well into the overall story, the Eternal World IS the overall story, as I see it. I think it was a pretty reasonable 'catalyst' for everything that went on.

For reference, I favour the routes in the following order:

Misaki > Akane > Nagamori > Nanase (Nanase's H scene was the best in the game, though =p) > Mio > Shiina. Not including Hikami's route in that, because I don't think it really counts. =p