Let m just clear out some misconceptions. It is true that Oda is always working a few chapters in advance of what he already made. But this extra chapters that are indeed complete, but not published, aren't there in order to get published if something ever happens to Oda. He participates in every publishing issue actively, there aren't any chapters that he sends to publish in advance. All the complete chapters are there to guarantee continuity of content. They're there to give the artist and the magazine an idea of where to end and begin each new chapter, in order to get the most content out of a week's publishing and still give a sufficient cliff-hanger to prepare the audience for the next week. The chapters in advance are for marketing and logistic, they allow the magazine and the artist to give a more well rounded chapter, when it does commence, and advertise in previous publications in expectation of the next chapter. Don't think Oda simple shows up in the morning of the magazine with a finished chapter and completely no preparation. The magazine knows in advance what he wants to publish, up to a few unpublished chapters, and then they work together to launch the actual, publication-ready, complete chapter.
So if something happens to Oda, or if he as a scheduled (with the magazine) break, the so called "complete" chapters won't be published, because they're only complete in terms of content and quality, not in terms of presentation to a reader. When he does prepare in advance for a longer break, it's an active work on his part, where he churns out chapters at a faster and unsustainable rate, they don't fish out a "complete" chapter from the stock he has.
