User blog comment:Omega natsu2/Fairy Tail Zero: Night of Embarking, Review./@comment-24102956-20140914172712/@comment-7418318-20140914180418

And that Mashima didn't show her grave in the manga yet is proof to you that she's still alive?

Apart from Mashima just not showing the grave (either to keep the mystery alive for a bit longer or because he didn't feel the need for it), there's also the possibility that there is no grave to begin with. There are fearsome predators living on that island. If one of them thought of Zeira's corpse as an easy snack before Mavis got a chance to bury her, there's nothing that a little girl without any magic could possibly do about it.