[morwen edhelwen]

there was an implication in the poem that she died years later. Notice how she's not mentioned in the last stanza. I think the "earth's daughter" thing is a reference to origins. Humans originated on Earth; the Elves are from Valinor- no longer "part of Earth"- but separate from the visible world, and no human, hobbit, or non-Elf/Maia/Vala can go there without special permission. I read the original 1934 poem somewhere on theOneRing.net, which ends with her rejoining her family, parents, siblings, cousins etc, and its implied that she was only briefly tempted to leave.