[Man-of-the-Wold]

First some potential errors above, for which I throw in my own two bits, which is what we do here.

The Last Ship into the West is the one that Círdan would captain. When, or if, that has happened is unclear, but it was probably at least a good while after the beginning of the Fourth Age -- probably later than the Legolas/Gimli sailing and thus, after the deaths of Aragorn and Arwen, though likely not too long thereafter.

There were still hundreds if not thousands of tarrying Eldar in Lindon, Rivendell and elsewhere, not to mention the Silvan Elves of Wilderland, who might all have longing and passage to Eldamar, as Nandor-Teleri.

I've imagined, assuming it didn't take too long, that Celeborn (and possibly Thranduîl) would join Círdan in the Last Ship, to sort of nicely conclude the Sindarin presence in Middle-Earth, that was caused by Thingol's encounter with Melian so long before in Nan Elmoth.

As for Elladan and Elrohir, they did, of course, have the option of being Men or Elves like their sister Arwen. That choice, however, was not indefinite, but it depended on when their father departed Middle-Earth. I don't necessarily believe that they absolutely had to leave on the very ship that Elrond took, but possibly. Probably, they would have had to do so within a grown Dúnedain's remainig lifetime at the longest.

Regardless, I have always had the impression, as noted by certain Tolkien commentators, that they end up staying and being mortal Men. One gets an impression that they are drawn toward that side of their ancestry and to Middle-Earth for whatever reasons, and the lack of any indication of their leaving is telling, although Tolkien may have seen it as too much of a footnote to be worth the trouble of belaboring. In any other words, it may not matter, except that if they stayed, then it is reasonable to imagine them marrying and having children, which is cool.

There is also the implication that Arwen gave up her seat on the boat for Frodo as she notes in her farewell to him, and so I've speculated that Bilbo's and Sam's (eventual) passages possibly correspond with ones surrendered by the brethren.

One interesting thing I've read is that Frodo, Bilbo and Sam did not necessarily gain everlasting life by going to Eldamar, but only the ability to live for quite some time and then die eventually at peace, because otherwise having born the ring tormented their hearts & spirits and interrupted their normal life cycle anyway. Witness that neither Gollum nor Bilbo actually dies after losing the ring, even if natural aging resumes.