[Pitchwife]

Welcome to the Downs, elbenprincess! The spelling of your name makes me wonder - is that a typo, or are you German, too (like me)? Anyway, enjoy being dead!

The Might has already summed up everything I was going to say very nicely, but I'd like to throw in my two cents about the question of Lingering/Fading and the fourth cycle.
Since we don't meet any bodily visible Elves anymore in our days (except for rare sightings of the kind you mention in your post), we can, I think, safely assume that they're all well into their fourth cycle by now.
Now Tolkien once said (somewhere in the Letters, IIRC) that we're now in the Seventh Age, each age covering something between 2000 and 3000 years (they were longer in the beginning, but have become shorter over time), which would mean that about 6000-7500 years (three ages à 2000-2500 years each) have passed since the War of the Ring and the end of the Third Age. Combining this with the observation that all Elves, even those who were young in Frodo's time, seem to have Faded completely in the meantime, it looks like the process of Fading, too, has accelerated, and they now reach their fourth cycle in much less than 10,000 years - probably due to the growing dominion of Men, I suppose.

As for the Elves in Valinor, that's a different matter - I wouldn't expect them to age in the same way as the Lingerers in Middle-earth, especially since Valinor has been removed from the circles of the world, and certainly not to Fade.

Like TM said, nice question, and not dumb at all !