[Guinevere]

Here is the exact quote from Tolkien's letters, that Ibri was referring to:

from letter # 325:

-As for Frodo or other mortals, they could only dwell in Aman for a limited time - whether brief or long. The Valar had neither the power nor the right to confer "immortality" upon them. Their sojorn was a "purgatory", but one of peace and healing and they would eventually pass away (die at their own desire and of free will) to destinations of which the Elves knew nothing.

even more detailed, concerning Frodo is letter # 246:

Frodo was sent or allowed to pass over Sea to heal him - if that could be done, before he died. He would have eventually to "pass away": no mortal could, or can, abide for ever on earth, or within Time. So he went both to a purgatory and to a reward, for a while: a period of reflection and peace and a gaining of a truer understanding of his position in littleness and in greatness, spent still in Time amid the natural beauty of "Arda Unmarred", the Earth unspoiled by evil.

In letter #154 Sam and Gimli are also mentioned:

But in this story it is supposed that there may be certain rare exceptions or accommodations (legitimately supposed? there always seem to be exceptions); and so certain 'mortals', who have played some great part in Elvish affairs, may pass with the Elves to Elvenhome. Thus Frodo (by the express gift of Arwen) and Bilbo, and eventually Sam (as adumbrated by Frodo); and as a unique exception Gimli the Dwarf, as friend of Legolas and 'servant' of Galadriel.

ConcerningTuor, nobody knows for certain what really happened to him:

...and with Idril Celebrindal he set sail into the sunset and the West, and came no more into any tale or song. But in after days it was sung that Tuor alone of mortal Men was numbered among the elder race, and was joined with the Noldor, whom he loved; and his fate is sundered from the fate of Men.

As Tolkien wrote in letter #153:

Tuor weds Idril the daughter of Turgon King of Gondolin; and "it is supposed" (not stated) that he as a unique exception receives the Elvish limited"immortality": an exception either way.

Tolkien left this deliberatly open!