Exactly! You don't set out to commit an adulterous act. Ideally you marry the woman according to God's law, and then--because of the covenants that you've made and the divine instructions that you've followed--the act becomes not adulterous, but holy.
There are, in the divine economy, tremendously significant acts that are not inherently evil but can nonetheless get us in an awful lot of trouble when we presume to perform those acts outside of divinely-imposed restrictions on time, place, and manner. Yes, in the eternal scheme of things, Adam and Eve had to partake of the fruit. They (well, Adam, per the Biblical narrative) had been told by God not to eat of the fruit and that doing so would result in their deaths; and then it seems they were out of God's direct presence for a while (which is why the serpent was able to effectively tempt them). But--and this is hinted at in the Bible, but more explicitly laid out in LDS thought--they were also expecting God to come back at some point. They were expecting further light and knowledge and instruction from Him. If Adam and Eve had been perfectly obedient, then at some point God would have had to have lifted His previous temporal instruction to not partake of the fruit if He expected them to fulfill His other commandment to multiply and replenish the earth. The only way to synthesize these commands is to read God's prohibition not as "don't partake of the fruit ever", but "don't partake of the fruit yet".
Put another way: The problem wasn't partaking of the fruit per se; the problem was that they partook of the fruit because Satan deceived them (well, Eve) into seeking god-like powers without developing God-like faith, God-like patience, God-like loyalty, or God-like obedience.
I frankly don't know. I accept Adam and Eve as historical figures, because as resurrected beings they have appeared to latter-day prophets. I believe they were the first individuals God saw fit to deem as fully "human" and having a full divine potential. I believe that they were incapable of having seed who were cognitively and intellectually and spiritually "human" in the same sense they were, until they made certain decisions and covenants before God that had eternal consequences for themselves and for their posterity (which at this point includes every human currently living). I believe that the essentials of those decisions and covenants are memorialized through some mixture of historicity, allegory, and symbolism in the modern temple endowment; and I believe that the narrative of Adam and Eve as a whole encapsulates the same journey (Presence, innocence, loss of innocence, estrangement, faith, covenant, reconciliation, redemption, salvation, return to Presence) that all humans must take.
Beyond that, I am not ideologically committed. I am aware that some (not all) LDS leaders--including some very vocal ones--have chosen to interpret the Genesis/Moses/LDS temple narrative as a literal recounting of the creation of earth and the fall of humankind and are adamant about concepts like young-earth creationism, no proto-humans (or "pre-Adamites"), no evolution, and no death of any living creature prior to the Fall. I am also aware that other LDS leaders have been less devoted to these ideas and, in some cases, have even embraced some degree of human evolution. And I understand that the fossil and archaeological record and known science--while ambiguous in a lot of ways--seem pretty darned conclusive that there were plants, animals, and even homo sapiens living and reproducing and dying (and in homo sapiens' case, engaging in social and ritual activities and caring for their sick and burying their dead) well before the period when the scriptures tell us Adam walked the earth. I am also aware that the Biblical creation and fall narrative is rife with, and responsive to, a variety of creation myths and literary and liturgical thematic tropes from all around the ancient Near East; and that when we read without understanding that context (and particularly when we try to read it as "history" in the modern western sense of the word) we run the risk of missing the entire point.
Which is all a very long way of saying: I don't know what the animals were doing before or during Adam's time in the garden or how they were affected by the fall, and I frankly don't find it terribly germane (other than the literarily tragic quality of Adam and Eve being clothed, after their fall, with the skins of [dead] animals).
The thing is, Elder Smith did in other instances speak of it as a "sin". Example:
It is most natural and just that he who commits the wrong should pay the penalty--atone for his wrongdoing. Therefore, when Adam was the transgressor of the law, justice demanded that he, and none else, should answer for the sin and pay the penalty with his life. But Adam, in breaking the law, himself became subject to the curse, and being under the curse could not atone for, or undo, what he had done. [Joseph Fielding Smith, Doctrines of Salvation 1:126, emphasis added.]
So what's going on here?
One of the nuances of Mormon thought that you may be missing, is that some LDS leaders have tried to draw a distinction between the concept of a sin versus the concept of a transgression. The concept here is that a sin constitutes a violation of an eternal and universal standard, whereas a transgression is simply a violation of a commandment of God that may be temporary in nature. For example, a follower of Jehovah would have been prohibited from eating pork in Mosaic times; a follower of Jehovah is under no such prohibition today. Under this line of thinking, an Israelite who ate pork would have transgressed, but wouldn't have necessarily sinned.
The problem with this dichotomy is that we Latter-day Saints aren't always very careful about observing and maintaining that distinction (as Elder Smith himself demonstrates), and our scriptures don't seem to recognize it at all. Outside the context of discussions about the Fall, we tend to use "sin" and "transgression" interchangeably. And the more you scrutinize it, the more questions it begs as you start hair splitting over definitions ("adultery" is always wrong and a sin, "sex" is wrong only outside of marriage and is therefore a transgression, but sex outside of marriage is adultery, which we already decided is a sin, so which is it? A sin, or a transgression? And our proselytizing materials used to define "sin" as "to knowingly disobey God"--our online Gospel Topics guide still effectively does--which doesn't recognize that sin/transgression distinction at all).
But more to the point, both sins and transgressions result in alienation from God and, in the absence of repentance and Christ's responsive grace, can only result in damnation.
Both sins and transgressions are wrongful. Adam and Eve's partaking of the fruit was absolutely wrongful because they did something God told them not to do. They did not make "the right decision". They should have waited on God, rather than acting out of ambition seeded by an enemy who had rebelled against God and was seeking to co-opt Adam's and Eve's loyalty for him own aggrandizement.
Now, we don't need to perseverate too much on the wrongfulness of their action, because (frankly) certain other religions do quite enough of that; and the Fall narrative has been used to justify an awful lot of misogyny and oppression and even violence over the years. The fruit was going to be eaten sooner or later anyways--from the perspective of the alienation from God that we, as Adam and Eve's posterity, experience, it makes little difference whether Adam and Eve ate out of disobedience or obedience; because the result for us (susceptibility to sin and death) is the same either way. God turned everything to His greater purposes, and there's little point to or need for a lot of flagellation or finger-pointing. So, for better or for worse, Latter-day Saint rhetoric has tended to give Adam and Eve a sort of pass for their conduct. But when you get into the nitty gritty of the scriptural account--the act still occurred in a context of disobedience and was, therefore, wrongful.
It is perhaps worth noting here that a) we shouldn't assume that just because one person didn't say something automatically means they didn't know it; and b) because of all the complexities and allusions and layers of the Genesis text as mentioned above, I don't think the sort of hyper-textual historicist scrutiny that you use to analyze Genesis 3:7 is likely to yield particularly reliable insights about either history or theology.
That said: I do partially agree with you here in that I think the verbiage in the Moses verses you cite is infelicitous; and it begs questions about whether Joseph Smith offered a sub-optimal translation (he was known to tinker with the verbiage of the sacred texts he had generated for decades after producing them) or whether Adam and Eve still, at that point, weren't comprehending the full nature of what had happened or the nuances and philosophical implications of the interplay between "sin" and "transgression" and "wrongfulness". (Or maybe Adam just never dreamed that a bunch of lawyers and textualists would be nit-picking his exclamations this closely!)
Whatever Adam and Eve may have said in their exuberance at learning that God Himself would make an atonement so that they would not be be cast off forever, disobeying God is always wrong. Always. Latter-day Saints understand this principle, and seek to embody it in the way they live their lives; and it tends to raise our hackles when outsiders seem to be trying drive us into either the Scylla of preaching a works-based soteriology or the Charybdis of lionizing and glorifying outright disobedience.