How does the 🧝Elves leaving the Grey Havens to the Undying Lands work?

[BalrogRingDestroyer]

I recall from the Silmarallion that they were more or less kicked out, or the Noldor were, for their Kinslaying and all of that. On the other hand, I recall that by the Third Age, Elves were going through the Shire, heading West to the Grey Havens to depart Middle Earth, never to return. Eventually, even Elrond, Galadriel, Celeborn, and finally I think Legolas left Middle Earth. I read that the ones that kept staying would eventually fade until they couldn't be seen. (Did that happen to Thrandil, who, unlike his son, was never reported as leaving?)

So what started allowing the Elves, even the Noldor, to leave in the Third Age, and what finally got Galadriel and Elrond to leave? (Was it, in their case, that the fairness of their lands, held up by the power of the Three Rings, died when the One Ring died, and hence they felt they had nothing worth sticking around for anymore?)

 

[Áramandë Amaniel]

I believe Galadriel would have been pardoned by the Valar when she resisted the call of the One Ring:

"I will diminish, and go into the West, and remain Galadriel."

Galadriel was of the Noldorin House of Finarfin and left Valinor for Middle-earth (thus falling under the Doom of Mandos and becoming one of the Exiles). However, this said Doom was lifted after the End of the First Age post-War of Wrath, and by the Third Age it would have been possible for her to return.

As for Legolas, he built his own ship to sail with Gimli to the Undying Lands.

Celeborn possibly left after Galadriel and Elrond did, seeing how the Realm of Lothlórien had faded by then without the preserving power of the Three Elven Rings.

As for the ones who kept staying in Middle-earth, it links to the concept of "the fading of the Elves", where in essence the fëa (spirit) would "consume" the hröa (body). Just as the Secondborn fear death, the Firstborn fear fading. In the Outer Lands that have been marred by the Darkness of Morgoth, the aging of the fëa would differ vastly from the aging of the hröa (unlike in Valinor where the aging rate would be the same).

In such cases (I am not exactly sure about this theory, and it seems Tolkien himself wasn't either) the fëa would probably refuse the summons to Mandos, and thus remain within the Outer Lands as a disembodied spirit, powerless without the hröa.

The concept of fading for the Eldar from the end of the Third Age onwards seems to be potentially inferable also from the Doom of Mandos:

"And those that endure in Middle-earth and come not to Mandos shall grow weary of the world as with a great burden, and shall wane..."

In any case, they would have rather left for the Blessed Realm than risk fading in the Outer Lands.​

 

[Elthir]

Not that anyone said otherwise, but we also have a textual example of Elves sailing in the Black Years:
" . . . which the Elves call the Days of Flight. In that time many of the Elves of Middle-earth fled to Lindon and thence over the seas never to return; and many were destroyed by Sauron and his servants. But in Lindon Gil-galad still maintained his power . . ."

So I'd say we are in the Second Age here. And other historical mentions that occur well before The War of the Ring include the Silvan Elves of Lorien sailing [Amroth, Nimrodel and so on], and Celebrian sailing over Sea in Third Age 2510.