Despite the fact that I still often say "Aeris," I don't think it's up to reasonable dispute that "Aerith" is closer to the original intent of the developers. How often are the names of major characters changed in later appearances?

Daniel, you seem incredibly confused by how game development works, and your repeated insistence that "Aeris" was not a mistake is either ridiculously pedantic or delusional. You admit that "this guy are sick" was a mistake, making your insistence that one particular name was not a mistake entirely arbitrary. The localization team can and frequently do mess up, in a variety of games. They can mess up in big ways and small ways (I mean, have you even played FFT? That translation was abysmal). The name is not, in fact, "plastered" all over the game, but appears once when you first name her, and then it's a variable for the rest of the game. The member of the localization team that translated the Japanese to "Aeris" made a mistake, in that they differed from the original intent of the developers who named her. The mistake was so obvious that many Western players still referred to her as "Aerith" even before SE officially revised her English name. It was not caught because the localization wasn't checked over with a fine tooth comb in those days; you can find plenty of contemporary games riddled with translation errors.

And most importantly, this was a mistake that was admitted by SE, who officially revised the name the character's future appearances. That should really settle the debate.

If you want to keep calling her "Aeris" because that's what you got used to, that's fine. Nobody would take issue with that, and I still do sometimes as well. Hell, you can still argue it's her "original" English name, because that just seems like a pointless semantic dispute that will vary depending on how you define "original" in that context. But "Aeris" was indisputably a mistranslation.