This is heartbreaking and wonderful all at once. (I love stories about Mandos; they are [one of] my weaknesses in Silmdom. ;) And, imho, not enough authors take advantage of the idea of Elves being able to resist the call of Mandos. Here, you use it to wonderful effect to show the grief of death, not in the loss of life, but in the loss of connection to what is left behind. These lines were simple and yet brilliant:

“Without me,” he said flatly.

“Yes, without you.”

for showing this first realization and, also I thought the very human urge to be remembered and grieved by loved ones, for some impact on their lives to marked one's passing.

An intelligent and disquieting story, IgB, filled with existential creepiness!  Nicely done.

Your Námo is a mix of the benign and coldly detached, but I'd be hard pressed to call him compassionate.  You've done a nice job in portraying the god of death (as I've noted elsewhere, hints of Hades and Anubis invaribly spring to mind when I think of Námo) as remote yet wise.

"White oblivion" is a fantastic bit of wordcraft here.  I immediately visualize the afterlife-as-white-nothingness as portrayed in films.

You convey an excellent sense of the peaceful combined with the strange here, not only with the white space and the deceased elf with his unraveling threads back to his former life, but also with the concept of a "configured spirit" burning its memories away to the small timeless flame. The prospect of dreaming oneself into a blank slate is a  frightening one, the promise of "pleasant dreams" notwithstanding.
 

This is a fascinating, and vey disturbing, look at Mandos.  Death may not be permanent for Elves, but you do a nice job of showing it's far from painless - and some of their losses (such as the separation from any family remaining in Middle earth) may be permanent.