I’ll give you a short version and a long version of each point.
Short version:

We do not believe in the Nicene Creed version of the Trinity.
We do believe in the virgin birth of Christ.
We believe that none of our doctrine conflicts with anything in the Bible and that the other books help clarify the interpretations that weren’t made as clear in the Bible.
It’s true that there are other groups recognized as Mormons that still practice polygamy (Fundamentalist Mormons). I should probably add that clarification point to the main post. To be honest, I kinda forgot about them. Oops!
The rest about plural wives I’ll leave for the long version below.
On marriage beyond this life or as we call it, eternal marriage. That verse we interpret as meaning that there will be no marriages performed in the resurrection, not that there will be no marriages.
You can buy almost everything our church sells on https://store.lds.org/. You don’t need to be a member, but you do need to make a free account.

Long version:
Our version of the Trinity we commonly call the Godhead. We believe in God, The Eternal Father, The Son (Jesus Christ), and The Holy Ghost. We think of them as being one in every sense except physically. That the Son was born in the flesh, and gained a physical body like the Father. When we use the term “God” we are referring to either The Father or The Son or even both (or sometimes all three). In addition, we also sometimes refer to Jesus as The Father because he has taken upon him our sins in effect making him The Father and The Son.
We don’t feel like Christian must only refer to those who believe in the Nicene Creed version of the Trinity because the Bible existed before the Nicene Creed, and there were Christians mentioned in the Bible as being Christian who differed in their understanding of who and what God is. (They certainly didn’t all believe in the Nicene Creed version before it was ever made, though maybe some did. Yet they were Christians.)

We have a slightly different belief on the form God chooses to take from that written in the Nicene Creed, but I don’t think the difference is great enough to detract from the fact that we believe Jesus Christ is our Lord and Savior, that he was born from the virgin Mary, that he suffered for our sins, died for us, took up his body and lived again on the third day, and that it is only through him that we can be saved. Ultimately, I guess we just believe the similarities vastly outweigh the differences.

As far as our extra books, we believe the Bible is the Word of God (and we add as far as it was translated correctly, since certain concepts don’t translate well to other languages from the original Hebrew, Aramaic, and Greek) but not all the word of God. Specifically, that God isn’t done speaking, and he never will be. Originally, the books in the Bible were separate. They were not written in the order they’re in, and there were many books that were decided of dubious enough origin that they weren’t included. (Such as the Apocrypha) But the people who would know which books were genuine were long dead.

We believe that God knew this would happen, and so commanded his prophets in other nations to write the Gospel and its doctrines as well so that we would have multiple sources to look to on any doctrinal topics that were fuzzy. And we believe that this falls in line with 2 Corinthians 13:1 where it says "In the mouth of two or three witnesses shall every word be established."

Most if not all of our doctrines can be found in both the Bible and the Book of Mormon. The Doctrine and Covenants contains more details than anything else. Since we believe in modern prophets, these were direct revelation to early church members spelling out details on meanings of other scriptures, and what to do about specific circumstances that they queried the Lord about.

The Pearl of Great Price really only contains 5 things:
Some translations of extra records that came from Moses and Abraham, an extra piece of scripture that we believe went missing from Matthew Chapters 23 and 24, The story of Joseph Smith’s literal encounter with God the Eternal Father, and Jesus Christ in the 1800s, and a short summation of our beliefs called our Articles of Faith.

For plural marriage, I look at it from a somewhat different perspective. In the Bible, it was indeed Kings who had multiple wives, and I that while their personal desires might have had something to do with it, who else could afford multiple wives? Keeping in mind that I only have my one wife, and so I have to guess as to what it must have been like, keeping one wife happy is hard enough as it is. And while from the outside looking in, we might imagine it like a harem of sorts. I suspect that our imaginations are far from the truth. I suspect it was more trouble than it would’ve been worth, and that if they didn’t honestly believe it was revelation from God, most of the men would’ve tapped out early in, let alone the women.

Again, this is all speculation on my part into the inner minds of the early members of my church. There definitely were many men who were killed in the early days of our church (At one point it was legal to kill us in Missouri), and as a result, there were many more women than men and if they wanted to maximize children without having sex outside of marriage, polygamy would’ve been the way to do it.
From a doctrinal perspective, all I have to go on is the few scriptures that touch on it, and the words of our modern day prophets.

Jacob 2:24-30 (https://www.lds.org/scriptures/bofm/jacob/2.html),
Doctrine and Covenants Section 132:35-39 (https://www.lds.org/scriptures/dc-testament/dc/132),
Official Declaration 1 (https://www.lds.org/scriptures/dc-testament/od/1)

Assuming from the perspective of a person who isn’t a member of our church that The Book of Mormon was made up in some way, why on earth would they write Jacob 2 at all if their intent was to dive right into plural marriage afterwards? Again, trying to look at it at least a little objectively (which of course I’m not, but hey, I’m trying. :)) it seems to me that it was honestly intended to be a temporary measure if it was needed because of population decline.