[Alcuin]

Mandos did not make rules: He enforced them. And he did that aggressively.

From Silmarillion, “Of Beren and Lúthien”, after Lúthien sang to Mandos in his halls of the suffering of Elves and Men, moving him to pity “who never before was so moved, nor has been since”:

Mandos had no power to withhold the spirits of Men that were dead within the confines of the world, after their time of waiting; nor could he change the fates of the Children of Ilúvatar.

Mandos couldn’t change the rules. In the debate over Eärendil and Elwing,

Mandos spoke concerning [Eärendil’s] fate; and he said: “Shall mortal Man step living upon the undying lands, and yet live?” But Ulmo said: “…is … Eärendil Tuor’s son of the line of Hador, or the son of Idril, Turgon’s daughter, of the Elven-house of Finwë?”…

…Manwë … said: “In this matter the power of doom is given to me. The peril that he ventured for love of the Two Kindreds shall not fall upon Eärendil, nor shall it fall upon Elwing his wife, who entered into peril for love of him; but they shall not walk again ever among Elves or Men in the Outer Lands. And this is my decree concerning them: to Eärendil and to Elwing, and to their sons, shall be given leave each to choose freely to which kindred their fates shall be joined, and under which kindred they shall be judged.”

When this debate takes place, Dior, Elwing’s father, is already dead. So are her brothers, Eluréd and Eluchíl. Mandos has already seen and knows what has happened to their spirits: If they’ve left Arda, he knows it and can report it, but if they’ve remained in Arda, he knows and will report that. His speech indicates they’ve all three departed from his halls and from Arda to wherever the souls of Men go.

The Problem of the Half-Elven had not yet been debated by the Valar. It didn’t require resolution until Eärendil arrived on the shores of Eldamar with Elwing and the Silmaril. Besides, Tolkien’s imaginary “history” is, like real history, the records of rulers. There’s no reason not to believe that there were other unions of Elves and Men (like Imrazôr and Mithrellas), just no other unions among the ruling houses: The only unions in the ruling houses were Beren and Lúthien, who both returned to Middle-earth as Mortals, and Tuor and Idril.

If Dior is Mortal – not “Man”, because physically he really is Half-Elven, and his wife is Elven and so “immortal” (longevial with Arda), then Elwing his daughter is in the same position as Eärendil: “Half-Mortal”. Half-Mortal may be a better term for this debate than “Half-Elven”.

Mandos enters into the debate about Eärendil and Elwing with the knowledge that Dior, Eluréd, and Eluchíl arrived in his halls and then departed Arda; otherwise, he cannot defend his position in the debate.

The key passage is this:

The peril that he ventured for love of the Two Kindreds shall not fall upon Eärendil, nor shall it fall upon Elwing his wife, who entered into peril for love of him

Eärendil and Elwing knowingly took upon themselves the dreadful risk of punishment out of love. And in respect of their self-sacrifice, a one-time decision was made: They could choose that fate they would share: with Elves to remain in Arda, or with Men to depart from Arda. The same choice was offered to Elros and Elrond, and to Elrond’s children while he lived.

I worked out the math several years ago, and I think I’ve posted it here before. Only if Dior is Mortal is Elwing also Half-Mortal.

My points, though, are these:

1. Mandos knew what happened to Elwing’s father and brothers, who had already died.
2. He was bound to truthfully report this to the other Valar.
3. Mandos wasn’t making rules: He was carrying out Rules he had been given.
4. Mandos cannot change anyone’s Fate: Elves stay, Men leave. The only exception so far was Lúthien, and she left with Beren.
5. His speech implies that Lúthien’s son and grandsons left Arda, too.
6. Manwë made an exception because Eärendil and Elwing acted out of love and self-sacrifice.

Manwë, not Mandos, also made the exception for Lúthien.

Love and self-sacrifice are the missing ingredients so far in this thread. Remember, Tolkien is Christian and Catholic. Love and self-sacrifice are central themes in the Crucifixion. “Greater love hath no man than this, that a man lay down his life for his friends.”

Beren sacrificed his life for love of Lúthien; Lúthien sacrificed her life for love of Beren, then begged Mandos for mercy. Perhaps because she’s half-Maia, Mandos relents, but he still has to go to Manwë to accomplish this, because Mandos has no authority to change the Rules. “Manwë sought counsel in his inmost thought, where the will of Ilúvatar was revealed.” We would say that Manwë prayed and meditated. In this way the line of Lúthien descended among Men. That wasn’t Manwë’s will: it was Ilúvatar’s.

Eärendil sacrificed himself for love of Elves and Men. Elwing sacrificed herself for love of Eärendil. This time, Manwë doesn’t hesitate: he cuts off debate and says they get to choose.

Elrond’s children also get to choose among which people they will be numbered, Elves or Men. Perhaps Elros’ three sons and daughter also had this opportunity; but probably not. I agree with Gothmog, LoB that this is “the real problem with the Peredhil” for readers. I think a reasonable explanation is that it was Eru’s will that, at the end of the Third Age, Arwen sacrificed herself for love of Aragorn to re-ennoble the descendents of the Númenórean kings as the Fourth Age began, and refresh the line of Lúthien. (Arwen was Lúthien's granddaughter's granddaughter. Aragorn was separated from Lúthien by about sixty generations.) Giving Elrond’s children the same choice as their father enabled this.

These are not the only times love and self-sacrifice appear as major themes in Tolkien’s mythos. In the Lord of the Rings, Frodo and Sam both sacrifice themselves: Frodo for love of the Shire, Sam for love of Frodo. Gandalf sacrifices himself on the Bridge of Khazad-dûm to protect Aragorn and Boromir, who foolishly charged onto the bridge and would otherwise have been killed. Gil-galad and Elendil sacrificed themselves to save their peoples and destroy Sauron’s physical form. Amandil last Lord of Andúnië sacrificed himself to save to the Faithful Númenóreans before the Downfall of Númenor: Elendil and his sons and followers were spared. This is a reoccurring theme in Tolkien.