Was Man's Opportunity to Live Forever A One Time Only Opportunity?

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http://www.chabad.org/library/bible_cdo/aid/8167/jewish/Chapter-3.htm
Breishit/Genesis 3:22-24

Now the Lord God said, "Behold man has become like one of us, having the ability of knowing good and evil, and now, lest he stretch forth his hand and take also from the Tree of Life and eat and live forever."
And the Lord God sent him out of the Garden of Eden, to till the soil, whence he had been taken.
And He drove the man out, and He stationed from the east of the Garden of Eden the cherubim and the blade of the revolving sword, to guard the way to the Tree of Life.

Rashi understands that it was indeed in theory possible for Adam to live forever, had he eaten from the tree of life and explains why G-d wanted to deny him this opportunity.

And now, lest he stretch forth his hand, etc.: And if he were to live forever, he would be likely to mislead people to follow him and to say that he too is a deity (Gen. Rabbah 9:5). There are also Aggadic midrashim, but they cannot be reconciled with the simple meaning.

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There are some views in the Talmud that at Mount Sinai when Israel received the Torah they were granted the opportunity of never having to die had they not sinned with the Golden Calf.
Tractate Avoda Zara 5a as translated by Soncino

Does that mean then that if they had not sinned, they would have
been immune from death? But there are written [in the Torah] the chapter about the widow of a man
dying without issue, and the chapter about inheritances!19 — These were written conditionally. But
are conditional passages written [in the Torah]? — Certainly; for R. Simeon b. Lakish said:20 What
is the meaning of the verse, And it was evening and it was morning the sixth day?21 It teaches us that
the Holy One, blessed be He, made a condition with all creation, saying, If Israel will accept the
Torah all will be well, but if not, I will turn the world void and without form.
The following objection was then raised: ‘The verse, O that they had such a heart as this alway
that it may be well with them and their children22 cannot obviously refer to the abolition of the angel
of death, since the decree [of death] had already been made?23 It means therefore that the effect of
Israel's acceptance of the Torah would be that no nation or tongue could prevail against them, as it is
said, that it might be well with them and their children after them’?24 He [Resh Lakish] may be of
the same opinion as the following Tanna, for it is taught: R. Jose said, The Israelites accepted the
Torah only so that the Angel of Death should have no dominion over them, as it is said: I said ye are
gods [i.e, immortals] and all of you children of the Most High, now that you have spoilt your deeds,
ye shall indeed die like mortals.25 But against R. Jose, too, [it may be argued] that the verse that it
may be well with them and their children for ever holds out the promise of well-being but not of
deathlessness? — R. Jose may reply: The abolition of death is surely as desirable a kind of
well-being as you might wish for.
Footnotes
(19) Which takes the incidence of death for granted.
(20) Supra 3a.
(21) Gen. I, 31.
(22) Deut. V, 26.
(23) At the worship of the golden calf.
(24) How then could Resh Lakish hold that but for the golden calf worship Israel would have enjoyed physical
deathlessness?
(25) Ps. LXXXII, 6.

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According to Masechet Derech Eretz 1:18 There are 9 that reached the spiritual level of being exempt from the decree of death issued by G-d as a result of the sin of Adam and Chava {Eve} in the Garden of Eden.
Chanoch, Eliyahu {Elijah}, Mashiach {Messiah}, Eliezer the servant of Avraham {Abraham}, the black slave of King Tzidkiyahu (who saved the prophet Yirmiyahu{Jeremiah}), Chiram the King of Tzor {Tyre}, Yaavetz the grandson of Rabbi Yehuda Hanassi, Serach the daughter of Asher {granddaughter of Yaakov/Jacob}, and Batya the daughter of Pharaoh (that saved the infant, Moshe/Moses}. There are those that say take out Chiram the King of Tzor (Tyre} and put in, in place of him, Rabbi Yehoshua the son of Levi.

מסכתות קטנות מסכת דרך ארץ פרק א

הלכה יח
תשעה נכנסו בחייהם לגן עדן, ואילו הן, חנוך, אליהו, ומשיח, ואליעזר עבד אברהם, ועבד מלך הכושי, וחירם מלך צור, ויעבץ בן בנו של ר' יהודה הנשיא, וסרח בת אשר, ובתיה בת פרעה, ויש אומרים, הוצא חירם מלך צור, והכנס תחתיו ר' יהושע בן לוי.

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Sanhedrin 91b as translated by Soncino

‘Ulla opposed [two verses]. It is written, He will destroy death for ever, and the Lord God will
wipe away tears from all faces;12 whilst elsewhere it is written, For the child shall die an hundred
years old . . . there shall be no more thence an infant of days!13 — It is no difficulty: the one refers to
Jews, the other to heathens. But what business have heathens there?14 — [The reference is to] those
of whom it is written, and strangers shall stand and feed your flocks, and the sons of the alien shall
be your plowmen and your vinedressers.15
Footnotes
(12) Ibid. XXV, 9.
(13) Isa. LXV, 20. The order of the phrases has been reversed here.
(14) I.e., in the re-established state after the resurrection.
(15) Ibid. LXI, 5

The author of the footnotes felt this would all take place after the resurrection, but it appears to me one of the Rishonim, Rabbeinu Nissim, "the Ran" felt this would take place in the Messianic Era before the advent of the World to Come. Now if you associate the World to Come with the resurrection (a point, which I admit is open to debate for several reasons) it would turn out that at that the Jewish people who "make it" to the messianic era will become immune from death according to Ulla.
This is the words of Rabbeinu Nissim, "the Ran" in Hebrew:

דרשות הר"ן הדרוש החמישי בסגנון אחר

ונתברר זה מדברי רבותינו זכרונם לברכה כי זאת הפרשה עתידה ממה שאמר בסופה (ישעיה סא ה) ועמדו זרים ורעו צאנכם ובני נכר אכריכם וכורמיכם, ואמר שם (סנהדרין צא ב) עולא רמי, כתיב (ישעיה כה ח) בלע המות לנצח, וכתיב (שם סה כ) כי הנער בן מאה שנה ימות, לא קשיא כאן בישראל כאן באומות העולם, ואומות העולם מאי בעו התם, אמר אין דכתיב ועמדו זרים ורעו צאנכם וגו'. הנה הוקשה לעולא מה שאמר כי הנער בן מאה שנה ימות, ובמקום אחר אמר שלא תהיה המות לעולם, והעמידה באומות עובדי כוכבים ומזלות שהם יאריכו ימים כדי להיותם אכרים וכורמים לישראל, ולפי הנראה לדעת עולא, כי ישראל שיראה ימות משיחנו לא ימות לעולם, ולפיכך הוצרך להעמיד מקרא שכתוב כי הנער בן מאה שנה ימות באומות העולם. ונתברר מזה שהיה עולא סובר, שמה שהנביא מנבא בלע המות לנצח, הוא לימות משיחנו ולא לעולם הבא. ברוך ה' לעולם אמן ואמן.