None needed at all! Cynicism is half the reason I wrote it; I can't buy them just sailing into Valinor, either. But since I'm inordinately fond of the Tuor-becomes-immortal ending, I needed some reason to justify that with canon saying Eärendil was the first success. (Sad to say, I didn't actually think of the possibility until I came up with the story idea. Cue a light bulb going on in my head: "Oh! That's how it happened.")