Question on the Melchizedek Priesthood and the First Vision

In a passage presumably well-known to both Latter-day Saints and Evangelicals, God tells Moses, "Thou canst not see my face; for there shall no man see me, and live" (Exodus 33:20). Less-known by Evangelicals is a prominent LDS expansion on this story in Doctrine and Covenants 84, the revelation 'On Priesthood', which reads in part as follows:

And this greater priesthood [i.e., the Melchizedek Priesthood] administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God. Therefore, in the ordinances thereof, the power of godliness is manifest. And without the ordinances thereof, and the authority of the priesthood, the power of godliness is not manifest unto men in the flesh; for without this [i.e., the Melchizedek Priesthood] no man can see the face of God, even the Father, and live. Now this Moses plainly taught to the children of Israel in the wilderness, and sought diligently to sanctify his people that they might behold the face of God [i.e., by obtaining the required Melchizedek Priesthood]; but they hardened their hearts and could not endure his presence; therefore, the Lord in his wrath, for his anger was kindled against them, swore that they should not enter into his rest while in the wilderness, which rest is the fulness of his glory. Therefore, he took Moses out of their midst, and the Holy Priesthood [i.e., the Melchizedek Priesthood] also; and the lesser priesthood continued, which priesthood holdeth the key of the ministering of angels and the preparatory gospel (D&C 84:19-26; bold is my emphasis, bracketed italics are my glosses)

As this expansion on the account in Exodus runs, of the Israelites at that time only Moses had the Melchizedek Priesthood, which he had received from his father-in-law Jethro prior to the Exodus (D&C 84:6). This makes it odd that in Exodus 33:20, God explicitly tells Moses that Moses could not survive seeing his face, when Moses had the only thing required to see God's face. Now that I think about it, I'm curious: what then is the explanation for Exodus 33:20 in light of this priesthood revelation?

(And yes, I realize that the Joseph Smith Translation - Exodus 33:20 JST - alters the text so that God says to Moses: "Thou canst not see my face at this time, lest my anger be kindled against thee also, and I destroy thee, and thy people; for there shall no man among them see me at this time, and live, for they are exceeding sinful. And no sinful man hath at any time, neither shall there be any sinful man at any time, that shall see my face and live." But the LDS Church nevertheless canonized the Bible as it stands in the King James Version, not Joseph Smith's revision of the Bible, which frequently removes all sense from passages such as this one. Nor does the JST here actually give an explanation why Moses could not see God, other than God telling him, "I might get really mad at you and kill you.")

But that isn't what initially had me puzzled. According to this revelation, no man can, while living in mortality, behold God the Father and live - unless that man has the Melchizedek Priesthood, which carries with it the power to directly behold the Father. An alternative reading might indicate that certain priesthood ordinances enable a person, whether or not they themselves hold the priesthood, to behold God the Father and live. As will become apparent, the same difficulty I'll be asking about would have a close analogy under that reading as well, so it makes no difference. A second alternative reading is more popular (it is espoused, for instance, by the LDS apologetics organization FAIR) and needs consideration. In this alternative, the antecedent of "this" is not the priesthood but the 'power of godliness'. There are two reasons, I think, why this won't work. First, the text clearly indicates that the 'power of godliness' cannot be manifested unto men in the flesh except through the priesthood and its ordinances. Nothing in the text allows for the claim that the power of godliness can be temporarily manifested without the priesthood but can only be permanently manifested through the priesthood. First, this seems to ascribe a very limited power to God, as though he could only protect a person with his Spirit for a brief time before tiring. And furthermore, it makes nonsense out of the remainder of the narrative, since then God could easily have given the Israelites a brief glimpse of himself by bolstering them temporarily with his Spirit. But in the narrative, the lack of the Melchizedek Priesthood is given as an explanation why the Israelites could not glimpse God's face at all. A second reason why this alternative reading is inferior is that it does not match the structure of the verse in question at all. The first portion of the verse is a statement that without A (the Melchizedek Priesthood), one cannot have B (manifestation of the power of godliness). The second half of the verse is an explanation of why that is so: "for without this [A'], no man can see the face of God, even the Father, and live [B']". The structure here indicates that the manifestation of the power of godliness must be taken as inclusive of the possibility of a vision of the Father's face. And it is for that reason that the antecedent of "this" in A' must be A - the Melchizedek Priesthood. The alternative reading would work well if the "for" were an "and", since then the second half of the verse would be new additional information rather than explanation. But as an explanation of the first half (and, it seems, a constitutive explanation), the verse only seems to make sense if "this" has the Melchizedek Priesthood as its antecedent. The same sentiment also seems to be upheld in a later revelation indicating that only with the Melchizedek Priesthood can one "enjoy the communion and presence of God the Father" (D&C 107:19).

After the apostasy which Latter-day Saints believe to have happened sometime between the apostolic period and the Constantinian Revolution, the Melchizedek Priesthood was removed entirely from the earth, as was the Aaronic Priesthood. Neither of those priesthoods was available until much later. The account runs that the Aaronic Priesthood was restored by John the Baptist in May 1829 (see D&C 27:8), though note that John the Baptist was not mentioned in tellings of this story until 1835), on the same day in which Joseph Smith and Oliver Cowdery baptized one another in the Susquehanna River, a river of which I have many fond childhood memories. Not until some time after that did they receive the Melchizedek Priesthood from Peter, James, and John sometime between May 1829 and August 1830, perhaps in June 1829 (see D&C 27:12-13; but note that Joseph Smith also claimed that the Melchizedek Priesthood was first restored at a meeting on 6 June 1831; see Times and Seasons [1 February 1844], p. 416, which reprints the phrase - which is also extant in Joseph's own Manuscript History - that "the authority of the Melchisedec priesthood was manifested, and conferred for the first time upon several of the elders" at this meeting). In either case, neither priesthood was held by anyone at all between the apostasy - no later than the early fourth century - and May 1829. The Melchizedek Priesthood was definitely, according to the LDS understanding, completely absent from the earth from the time of the apostasy until no earlier than June 1829. And without the Melchizedek Priesthood in particular (and/or its ordinances), no one can behold God the Father face-to-face and live, according to the revelation given in D&C 84. Therefore, between the fourth century and June 1829, no mortal man could possibly have seen God the Father and survived the encounter, because the Melchizedek Priesthood - which was entirely absent during that period of time - is a necessary condition for survival under those circumstances.

However, according to the canonical account of the First Vision, which that account places in 1820 - which falls into that span of time - Joseph Smith was confronted by not only Jesus Christ, but also by God the Father (Joseph Smith - History 1:17). Crucial to the standard LDS understanding of the First Vision is the view that Joseph Smith, at that point in history, saw God the Father with his own two eyes and lived to tell the tale. If he did not, then either the account never transpired at all - which would require either shuttling it off into the realm of myth or denouncing it as a falsehood - or else the account was a vision in which he dreamt that he saw the Father and the Son, but neither actually appeared to him bodily. What this would mean is that the First Vision itself would be relatively uncontroversial - non-LDS Christians could easily grant that Joseph Smith fell asleep and had a dream while denying that it was an inspired message of any sort - but also that the First Vision was misrepresented by Joseph (at least, it seems unlikely that he would have gotten the same sort of reaction had he claimed to have dreamt about God) and that the First Vision would have little value in supporting LDS views of divine embodiment and the lack of common essence between the Father and the Son. (Actually, even if historically true in the waking world, it would do neither, since an incorporeal God could easily manifest himself in human form for the sake of a comprehensible theophany; and nothing about distinctness of persons or even distinct physical incarnations would be contrary to the Nicene assertion that the Father and the Son are of the same essence [ousia]. But that isn't the point here.)

So here's what's puzzling me. If we suppose that the Melchizedek Priesthood is necessary to behold God the Father while in the flesh and live; and we suppose that Joseph Smith could not have held the Melchizedek Priesthood in 1820; and we also suppose that Joseph Smith beheld God the Father while in the flesh in 1820 and lived - then it seems that we encounter a fundamental contradiction. (Note also that up to that time and afterwards, Joseph frequently lamented his sins, which may introduce an additional tension with Exodus 33:20 JST.) We could reduce it through some substitution:

1. If x lacks A, then x cannot do B.
2. x lacked A.
3. x did B.

(Obviously, x here is Joseph Smith, A is 'holds the Melchizedek Priesthood', and B is 'see God the Father while in the flesh and live'. If you're troubled by the fact that x eventually did obtain A, then feel free to mentally preface each statement with 'at t', where t is the spring of 1820, or whenever it was that Joseph Smith purportedly had his First Vision.) The contradiction seems clear, since (1) and (2) together entail ~(3): 'x did not do B'. But the problem is that it seems that faithful Latter-day Saints must be committed to all three propositions. One cannot reject (3), it seems, without rejecting LDS scripture, since (3) is contained in the Pearl of Great Price and is also part of the foundational LDS narrative. One cannot reject (2), it seems, because (2) is a part of the Church's narrative and is also bolstered by Doctrine and Covenants 27. And one cannot reject (1), it seems, because (1) is likewise contained in Doctrine and Covenants 84. So the three propositions contain an explicit contradiction; but it certainly seems that none of the three propositions can be denied without rejecting a significant portion of what Latter-day Saints regard as inspired scripture.

Now, perhaps I'm either reasoning wrongly or reading wrongly. But no explanation that I've seen so far seems satisfactory, which is why I feel strongly compelled to post this here in search of an answer. (I realize that it may look like I'm presenting an argument against LDS beliefs. And in a way I am, but that's because presenting an argument in a sort of 'test run' is how my mind works when I try to get to the bottom of things. I usually mull it over inside my head for a while as I formulate the argument and try to challenge my assumptions, and if that doesn't dismantle it in my eyes, I generally try it out on someone who's likely to disagree with the conclusion - or at least can play devil's advocate pretty well.) Can you see a way to resolve this issue?