User blog comment:B214/Chapter 499: Gray & Juvia/@comment-5303512-20160829121430/@comment-1359322-20160829234603

"Mashima can just change things as he continues writing or in this case, there's nothing that guarantees what Zeref told Natsu in the Tartaros arc will remain the same as he continues writing. He can change it anytime he wants."

This is the point. You're assuming that Mashima won't change anything at all. He'll change things to fit into the scenario he's writing in the end, there's nothing guaranteeing that Natsu and END will be a separate entity at all.