Actually, there's an easy distinction Sierra has a large 'e' whereas Shera has a small 'e' to denote that you drop the 'i' of Shi and simply state She as opposed to Shie.
I'm not saying that they are errors. I'm saying they are mistranslations. Deliberate ones, but mistranslations nontheless.As for FFVI - those were deliberate changes, too, rather than errors. 'Terra' instead of 'Tina', for example, is frequently explained: it simply sounds more exotic, more suited to a fantasy character. 'Sabin' instead of 'Mash' is pure aesthetics; while you can argue that 'Aerith' is no more stupid than 'Aeris', there's no denying that 'Mash' is simply a stupid name in English.
As for Mash... Eh, I don't think it's that dumb, but I know people with some very esoteric names.
In any case, the 'FF3' names are still not the official names just as 'FF3' is not the official name of the game. The prime source wins.





