I know that it must have been mistranslated because of the errors that are contained therein. Unless God made those mistakes. And I highly doubt that.
For the most part, we do have the contradictions covered, largely because of modern day prophets that are in contact with God.
Yes, we do use the King James Version of the Bible. And even though that contains many of the same contradictions, we've learned through modern revelations that it is the most correct of the various versions out there.
I was originally taught by a missionary, but I've gained a testimony of the truth through the power of the Holy Spirit. Much in the same way that you came to believe that Jesus Christ is your savior.
Yes, it is the fulness of the gospel, but it completes the gospel, it doesn't replace it. The Bible contains many aspects of the Christian faith, but our scriptures complete it. A lot of what we believe was originally going to be part of the Bible, but through various circumstances, the books were lost to the world.
Despite being full of contradictions, the Bible is still an essential part of salvation. Most of it is right and if we are guided by the Holy Ghost, we can decifer what is wrong.