Was Melian embodied like the Istari during her time in Doriath?

Question

In The Silmarillion, we get the story of Melian and Thingol; Melian, being a Maia, does not have a body that she is bound to, but rather was able to take on the shape and likeness of creatures of the world or the children of Iluvatar.

But...she bore a child. Lúthien Tinúviel, who was never described as being anything other than a (particularly beautiful) elf. This, as The Silmarillion might phrase it, is a thing strange to me; that a being using a body the way we use clothing could be capable of bringing forth one of the children of Iluvatar. The Maiar, after all, are not supposed to be able to create new life, yet that's very much what motherhood is. So...

Is there any indication in the text or letters or similar that Melian was 'incarnated' the way the Istari were? Was Lúthien a 'child by adoption' of Eru, as the dwarves were?

Answer 1

Yes.

A note in History of Middle-earth 12, The Peoples of Middle-earth, reads:

Melian alone of all those spirits assumed a bodily form, not only as a raiment but as a permanent habitation in form and powers like to the bodies of the Elves.

A further note in the Orcs material in History of Middle-earth 10, Morgoth's Ring, reads:

...by practising when embodied procreation they would (cf. Melian) [become] more and more earthbound, unable to return to spirit-state (even demon-form), until released by death (killing), and they would dwindle in force...

That's two definite statements, at two different times and in two different contexts, that Melian had been, or had become, permanently physically embodied, so it seems clear that this was Tolkien's conception of her state.

Answer 2

Melian was temporarily bound in her physical embodiment from when her daughter Lúthien was born until both her husband and daughter had died.

In a c.1959 text later titled "The Quendi Compared to Men", Tolkien discusses the known cases of elves who only had one child (which Tolkien says is rare), and there's a footnote with more information about Melian.

Melian assumed (as the Valar and Maiar could) "the raiment of the Children", the Incarnates, out of love for them. Only one of the greatest of the Eldar in their early vigour could have supported a union of that sort (unique in all known tales). But Melian, having in womanform borne a child after the manner of the Incarnate, desired to do this no more: by the birth of Lúthien she became enmeshed in "incarnation", unable to lay it aside while husband and child remained in Arda alive, and her powers of mind (especially foresight) became clouded by the body through which it must now always work. To have borne more children would still further have chained her and trammeled her, In the event, her daughter became mortal and eventually died, and her husband was slain; and she then cast off her "raiment" and left Middle-earth.
_{The Nature of Middle-earth, page 21, footnote}

Answer 3

Victim of Circumstance answered the embodiment part, so I'll just tackle the second question --

No, Luthien was not a "child by adoption" of Eru as the Dwarves were. Tolkien always refers to her as one of the Eldar / Eldalie and she would have shared their fate were it not for the intervention of Mandos (and permission of Iluvatar) - she was not of an unique race.