[William Cloud Hicklin]

and/or even swapped her "berth" on a ship with Frodo. I seem to remember she implied to Frodo that he could go West in her place.

Well, Tolkien expressly denied this last option.

It is not made explicit how she could arrange this. She could not of course just transfer her ticket on the boat like that! For any except those of Elvish race 'sailing West' was not permitted, and any exception required 'authority'....No doubt it was Gandalf who was the authority that accepted her plea. The Appendices show clearly that he was an emissary of the Valar, and virtually their plenipotentiary in accomplishing the plan against Sauron.

He then says something which bears on the matter of Arwen herself:

and she was not in direct communication with the Valar, especially not since her choice to become 'mortal'.

This is doubly interesting: the implication (reinforcing others) that by the time of Arwen's discussion with Frodo in the garden, the Choice was already operative (before Elrond's sailing); and also the implication that those of the Elvenkind were somehow in closer "communication" (whatever that means) than mortals.

What is meant is that it was Arwen who first thought of sending Frodo into the West, and put in a plea for him to Gandalf (direct or through Galadriel, or both), and she used her own renunciation of the right to go West as an argument. Her renunciation and suffering were related to and enmeshed with Frodo's: both were parts of a plan for the regeneration of the state of Men. Her prayer might therefore be specially effective, and her plan have a certain equity of exchange.

There is a trend in Tolkien, at least in his later reflections on his work, that 'exceptions' were granted specifically in those cases where a Divine Plan was involved: Melian's becoming incarnate and bearing a child; Luthien's becoming mortal and bearing a half-Elven child; Tuor's finding Gondolin, marrying an Elf and siring Earendil (and, we think, becoming an Elf himself); and Glorfindel's being released and re-embodied not just for his heroism, but specifically because his heroism ensured Earendil's survival. To this we might add the only other Noldorin Exile we know affirmatively to have been released from Mandos, Finrod: his sacrifice allowed the quest of the Silmaril to be achieved, and ultimately the "divine" strain in Men.