I believe what Manwe means is that any product of and Elf/Man marriage will inherit the Gift of Men, (ie 'Mortality' the true death and escape from the circles of the world) *BUT* this does not necessarily mean they will also have a merely mortal lifespan. Dior was about forty when he died, old enough to begin showing signs of aging but there is no mention of any such thing. Nor do Earendil or Elwing seem to have aged after attaining physical maturity.
IMO, and this *is* only my opinion, the Half-Elven are 'immortal' in the sense they do not age or die naturally *but* if they should be killed or chose to die from grief or weariness they will share the fate of their Mortal parent and pass beyond the circles of the world.
If true this would explain Arwen and the twins lifespans. According to the judgement of the Valar they were not required to make a final choice of their fate until their father left Middle Earth for Aman. If they wished to be counted among the Eldar they would go with him, if they remained behind they would be accounted among Men. Which means the twins have chosen to become mortal too as they do not accompany their father.
Note also that Arwen doesn't die of old age but of grief just as her foremother Luthien did the first time, there is no indication she ages at all.
I don't quite understand why everybody says Elros' children were not offered the choice too, where does it say that? What does seem clear from the genealogical chart in Unifinished Tales is that all three of Elros' sons chose to be counted as Men for they married and left descendants. His only daughter Tindomiel on the other hand left no descendants nor does she have a date of death - could that mean she chose to be of Elf kind and sailed West? Possibly Elros' children were also given a time limit to make their final choice, perhaps at the time of their father's death?
However it seems clear that theirs is the last generation of Half-Elven to be given the choice. Their children are Men, but still far longer lived than others of their kind. Which means Eldarion will also be accounted a Man from his birth but will have a longer than normal lifespan, even for a Dunedain.
As for the question of whether there are other Half-Elven, Mithrellas and Imrazor suggest there may be a few, but probably not many and mostly of relatively humble Sylvan ancestry.
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