The Hobbit Ring-Lord?

[Tuor of Gondolin]

"Would the Quest of the Ring end in everlasting victory (Frodo casts the ring/is pushed into the fire), everlasting defeat"
=============
Certainly not "everlasting" defeat. The valar, or, if necessary, Iluvatar would
have eventually intervened. My understanding of free will, and in particular the
pre-Christian Christian LOTR philosophy, is that this can lead to long periods
of "darkness" if evil is ascendant (much of the Second Age, for example) but that
eventually there would be a redress of events. Think of Gandalf's commment
somewhere that he would not have failed if anything survived which could
flourish (I don't recall the quote- obviously). And also Eru's comment to Melkor:

And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.

One could imagine, over several hundreds of years, and with some powerful, but hidden, maia emissaries, gradual unrest leading to Sauron's overthrow, thus
allowing free will of good and evil peoples, including a failure of good peoples to
effectiively unite and act leading to defeats- thus free will acts of good guys and
bad guys having a real effect, while still allowing for an eventual "redemption"
of Middle-earth.

==================
As for the Ringwraiths, it seems highly probable they would not have really
followed Frodo. (In Letters):

...they would have obeyed or feigned to obey any minor commands of his that did not interfere with their errand-laid upon them by Sauron, who still through their nine rings (which he held) had primary control of their wills.