[Man-of-the-Wold]

Arwen's choice was in effect made when her father left Middle-Earth, and she was not aboard, because she had wed a mortal, and so couldn't then leave. Arguably, her brothers might still have been able to chose otherwise, if having departed for Eldamar, say, within the time of an adult human lifespan following the passing of the Ringbearers; lets say Fourth Age 82 was the deadline, so-to-speak. (I for one believe that they did not, having come to relate to the fate of men)

So, when Arwen said she could no longer leave at Aragorn's death, it was true, besides what it would've meant to the sanctity of their marriage.

Clearly, Elros's children had no such choice, because Elros could not and did not go to Elvenhome. So, his choice was also to have his children receive the Gift of Men. The Valar, perhaps, did not wish to deny that Gift to Elrond's children, who would normally have had it, regardless of who their mother would be.

What is significant about Elros, Elrond and his children is that they did have a choice. The default rule, if not known at the time, was that any amount of mortal blood made one mortal. So, Dior, his children and Earendil would have all died a natural death, except for being killed, or given a choice.

And some physical features aside, Imrahil was no more "Elvish" than Faramir, despite having ancestors descended from a semi-Eldarin maiden, who had passed through Belfalas on her way to the West a thousand years earlier.