Is World 3 mostly sidequests?

After a little bit of reading up, sources seem to suggest that the lithograph hunt is all sidequest, despite it seemingly being part of the same story. Is it theoretically possible to go into the N-Zone, for the final battle, without Reina/Lenna? Or have I just fallen victim to some confusing article on the subject?

You have to go through that forest to get her. But that's the last thing you have to do, you get the airship right away and can go straight to the final dungeon.

World 3 is probably the world with the most optional stuff, yes.