1.) Tolkien kept changing his mind on which Elves could sail West.
In Letter 154 he clearly states that only the High Elves could sail West --
Elves such as Legolas would not have been able to do so. On the other
hand, Legolas does sail (Appendix B); and it is implied that Celeborn will also
(Prologue). Celeborn at this time is still conceived of as a Silvan Elf,
native to Lorien.
2.) In the so-called suppressed Epilogue (see Sauron Defeated =
History of Middle-earth, vol. X) Tolkien has Sam state that no ships are
sailing West these days: they've stopped. As the Epilogue was written
about the same time as the Prologue and the Appendices, this would imply that
Arwen's statement that no ships were sailing is literally true: "There is no
ship that would bear me hence." In that case, Arwen's argument is this:
while technically she might reverse her decision to die and go West, there
aren't any ships sailing, so the question is moot.
At any rate Aragorn is clearly of the opinion that she
could (at least technically) sail West.
Nowhere in the LotR does Tolkien say when exactly Arwen's
choice to become a mortal became irreversible. Tolkien made a statement
on it, after the fact, in Letter 153, namely that she made her choice when she
wed Aragorn. This is not compatible with what Tolkien wrote in Appendix A
I (v).
But, then again, Tolkien kept changing his mind on all of
these things, so who knows... You're entitled to make up your own
mind.
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Although Frodo -- mostly to spare the old wizard's feelings -- recorded another version of the story, at the entrance to Moria it was not Gandalf who solved the riddle of the inscription, or indeed who found the door.
