Middle Earth "afterlife"

[Legolas]

I still hold that Luthien's single, exceptional case doesn't warrant mentioning that - no more than Tuor's conversion in the oppisite direction. It's still misleading that one would say 'elves can sometimes choose mortality' - with the direct action of Eru, any beings' fundamental kind can be changed. You're right though, we can leave it at that.

Dior and his children were definitely half-elves. This is part of the significance of Elwing and Earendil's union, and their family. Dior, Elured, and Elurin were slain in Ruin of Doriath, leaving Elwing as the only descendent of Luthien and Beren. Elwing and Earendil were both half-elven, and given a choice after their great contributions in the War of Wrath, Elwing chose to be an elf, and Earendil followed as he did not want them to be separated.

With the deaths of Dior and his sons, Elwing and Earendil represented to the two remaining strains of elf + man, and by joining, these two special cases were narrowed down to one lineage which directly allowed Numenor to happen, and eventually Elendil and Gondor, and finally Aragorn who was able to hold off the attack of Sauron and usher in the 'Age of Man.'

Tolkien mentions this intentional significance (and the first point as well) in Letter #153.

Immortality and Mortality being the special gifts of God to the Eruhini (in whose conception and creation the Valar had no part at all) it must be assumed that no alteration of their fundamental kind could be effected by the Valar even in one case: the cases of Lúthien (and Túor) and the position of their descendants was a direct act of God. The entering into Men of the Elven-strain is indeed represented as part of a Divine Plan for the ennoblement of the Human Race, from the beginning destined to replace the Elves.