This was such a heart-ache - both for the reader and Fëanor ?

Oh, what a bittersweet view into Fëanor's POV.

I enjoyed your little details, such as "the small perfection of a garter snake curled up to sleep"

"A whiff of breeze awoke a singing shiver through the nearby reeds."

"all things in Aman were tailored to be of use. He had searched high and low for anything wild"

"For now, in the eternal spring of Valinor, it only reminded him of broken glass."

This is an interesting line of thought: "Sometimes, Fëanor wondered if their decision would have been different had Finwë been a simple fisherman who lived along the coast, or a woodsman deep in the forest;"

"He didn’t want anyone else to suffer, or to mar the beauty that surrounded him" — I really like fundamentally he's kind and considerate. 

And this, I fully relate to; so often there is a part of my mind that is aware of and noting beauty even in the most awful circumstances: 

"despite the grief, some part of his mind had still rejoiced in the faint grace of this silver dust sprinkled throughout heavens. His father had died because of him. It was a strange mercy that, in this pain, he had seen beauty."

A lovely, thoughtful, and yes, beautiful ficlet.