Who wrote...

[Gordis]

I agree that the triple emphasis on mortality points to Elves as authors.

Originally Posted by Sardy

The immortal elves, however, within their poetry very well may make use of the "artistic liberty" of redundently describing men as mortal, in a way poetically re-affirming their own immortality...

Yes, but if the Elves were only reaffirming their own immortality, why not mention at least once that Dwarves were also mortal? Maybe because (unlike Men) the Dwarves themselves seemed content with it and never strove for immortality?

Theoretically, however, Elves are supposed to see Death as a Gift for Men and envy it:

Messengers from Valinor said to Men: "The Eldar, you say, are unpunished, and even those who rebelled do not die. Yet that is to them neither reward nor punishment, but the fulfilment of their being. They cannot escape, and are bound to this world, never to leave it so long as it lasts, for its life is theirs. And you are punished for the rebellion of Men, you say, in which you had small part, and so it is that you die. But that was not at first appointed for a punishment. Thus you escape, and leave the world, and are not bound to it, in hope or in weariness. Which of us therefore should envy the others?"

So, maybe the undercurrent of this line is "Mortal men doomed to die who are never content with it and foolishly try to escape the Doom"?

Also maybe, the Elves considered the tweaking of the nature of Men (which even the Valar can't do) the most horrible and "unholy" effect of the Rings overall?

But if so, wouldn't it be better to write "Nine to the Mortal Men doomed to live?"