Tolkien's final three drafts of the Quenta Silmarillion conclusion (1937, 1951, c.1958)
In 1937, Tolkien wrote out a new version of the ending to the Silmarillion. (He jumped ahead a bit, as the rest of the 1937 draft cuts off before that point.) This draft included a specific line about the default fate of half-elven. In 1951 and c.1958, Tolkien made new typescripts of this chapter with some minor changes. This passage remained unchanged.
Now all those who have the blood of mortal Men, in whatever part, great or small, are mortal, unless other doom be granted to them; but in this matter the power of doom is given to me. This is my decree: to Earendel and to Elwing and to their sons shall be given leave each to choose freely under which kindred they shall be judged.
Quenta Silmarillion, 1937, 1951, and c.1958 drafts (The Lost Road and The War of the Ring)
Christopher opted to not use that line in the version of The Silmarillion that he published. He offered no explanation as to why, but my guess would be that he saw potential conflict between it and the Peter Hastings letter (see below) and so preferred to remain ambiguous.
Draft letter to Peter Hastings (1954)
In a 1954 draft letter to a catholic bookshop owner, who had inquired (among other things) about half-elves, Tolkien seems to imply here that the choice is a result of the union, and that Elrond's children are given a choice because of Elrond's marriage to Celebrían.
The view is that the Half-elven have a power of (irrevocable) choice, which may be delayed but not permanently, which kin's fate they will share. Elros chose to be a King and 'longaevus' but mortal, so all his descendants are mortal, and of a specially noble race, but with dwindling longevity: so Aragorn (who, however, has a greater life-span than his contemporaries, double, though not the original Númenórean treble, that of Men). Elrond chose to be among the Elves. His children – with a renewed Elvish strain, since their mother was Celebrían dtr. of Galadriel – have to make their choices.
September 1954 Draft Letter to Peter Hastings (The Letters of JRR Tolkien #153)
LotR Appendices (c.1954)
In Appendix A of The Lord of the Rings, Elrond's children are said to have a choice to become immortal specifically because of an extension of the special fate given to their father. One would think that if all half-elves had such a choice, they would have one as well given that their mother was an elf.
But to the children of Elrond a choice was also appointed: to pass with him from the circles of the world; or if they remained to become mortal and die in Middle-earth.
The Lord of the Rings, Appendix A
(Note though that in an earlier draft, "or if they remained" was "or if they wedded with one of Mankind".)
Similar statements are made elsewhere in the drafts of the appendices.
"Elvish Ages & Númenórean" (1965)
In some of Tolkien's late work discussing the aging of characters, he refers to the lives of half-elves prior to receiving the choice as a mortal (though slower) life, and uses that to calculate how much Elrond would have aged by the time that the choice happens.
Elrond. The “Half-elven” should age slower than ordinary Men, before the “doom” of the Valar was spoken. Probably at rate of 1 to 5 as for Elros, the only one who lived his life out as Half-elven. (Full growth being achieved at Elvish rate of 24 but reckoned in normal löar.)
Elrond was present (see LR I 256) at the fall of Thangorodrim. Eärendil his father wedded Elwing in FA 525, being then 23. Elrond may have been born about 527–530. He was thus at least 70 at the fall of Thangorodrim in c. FA 600. But this would be the [mortal] equivalent of 24 + 46/5 = approximately 33.
He was made Elven soon after, and would then slow down to the Elvish rate of 144 SY = 1 life-year. In SA 3441 he would be 56. In TA 100 he would be nearly 57, but still in youth (= mortal equivalent nearly 43).
"Elvish Ages & Númenórean", 1965 (The Nature of Middle-earth)
With the Peter Hastings draft letter as an outlier, the rest of Tolkien's work from the 1950s and 1960s seems to indicate a default state of half-elves being mortal, and that the choice was an exception, and not given to everyone.
However, none of this applies to Dior.
What many forget is that Dior is not a half-elf. He was born after Luthien was made mortal, so at the time of his birth, both of his parents were mortal.
Dior's three children Elwing, Elurin, and Elured are half-elven, but this is only because of their mother Nimloth, not because of their father.
