With respect to your questions regarding what constitutes Mormon Doctrine, your question is best answered by the Japanese “mu,” which means that the question is misinformed so it is better to withdraw the question. Like Judaism, and it appears earliest Christianity, there is neither “official Mormon Doctrine” nor council or creed that establishes such matters. Rather, there is a tradition of interpretation that is like the common law approach to deciding what constitutes the law. It is taken on a case-by-case basis guided by prior precedent of revelations, decisions and practices. So everything in the scriptures is “doctrine,” but of course that leaves open a lot of different approaches. It is well-settled that the doctrine of the Church is that Jesus is God’s Son and our Savior. What these basic affirmations mean is left open. It is basic that the Book of Mormon was translated by the gift and power of God. What that means is left open to a range of interpretation.
The question also assumes that somehow getting doctrine right is vitally important. To the extent it facilitates a saving and exalting relationship with God, it is important. But it isn’t important, for instance, to be able to formulate the relations of the divine persons in the Godhead correctly. If that were the standard, then only very articulate and very bright people could hope to be saved and exalted. I get the idea from the scriptures that it is more about a soft and loving heart than a correct idea in the head. In fact, taking such issues seriously enough to cause dissension over them is counter-productive to loving harmony int he community of Saints that is much more important in the end.
So LDS doctrine is open-textured. Such openness is necessitated by the ongoing revelation and the fact that we admit that there is a lot we don’t know and a new revelation might put what we thought we once knew in a new light. That Jesus is our Savior must be affirmed — that one must accept the penal theory of atonement is optional (and in my view wrong-headed).
It follows that there is no authoritative and comprehensive work on LDS doctrine except the scriptures. However, there seems to be a hierarchy of authoritative sources that one uses in the interpretative context to persuade what constitutes truth and obligation. First, the scriptures are accepted in all that they say. However, just as there are many different religions and creeds that derive from the Bible, there are many that are possible within a Mormon context. Next in authority I would place the uncanonized revelations of Joseph Smith and his successors. Next in authority are the Official Statements of the First Presidency. Next I would place the sermons of Joseph Smith and after that those of his successors. Next I would place LDS Church publication. Knowing this hierarchy of authority doesn’t give one an encyclopedia of Mormon Doctrine — but it gives one the sources necessary to engage in the interpretive dialog about what LDS will accept and take seriously.
Let me give an example of how this interpretive context may work. As should be obvious, the mere fact that you can find something in scripture that seems solid to you (e.g., Second Isaiah’s statements that there is only one God who is Yahweh) is not going to be definitive because that must be placed within the wider context of interpretive sources and revelatory authority. It so happens that these statements are made alongside the recognition of foreign gods and gods who are the sons (and daughters) of God in the council of Gods. These Old Testament texts will be read in context of Jesus’ revelations about the Son of God who is called “God” in scripture and is not identical to the Father whom Jesus also recognized as God. These crucial texts will also be placed in the context of Joseph Smith’s observations about the God of all other gods in his revelations. These revelations will be placed in the wider context of his sermons about the council of gods and so forth. In the end, there is a sense in which there is only one God, but not in the sense that the creeds have assumed (based largely on the interpretive context of the Greek metaphysics in which the Church Fathers wrote). There is a sense in which there are three in the Godhead. There is a sense in which there is a plurality of gods. Among Mormons, there is a good deal of disagreement over whether these other gods are subordinate to a Most High God, or there is an infinite regress etc. because such matters have not been fully clarified by revelation.
In the end, it turns out that getting doctrine right is not what is essential about being a Christian Mormon. What is essential is being open to love others and do whatever that entails. It is also vital to be open to hear God’s voice when he speaks — and most often that is to assist to discharge the duty to love others. While one could not be Mormon while fighting against the kingdom of God established through the Church, one could follow God with doubt about just about everything fundamental and no clear views on even fundamental “doctrinal issues.”
I don’t know if that is very satisfactory to you, but it is, I believe, an accurate assessment of how Mormons go about determining what to believe.