I just read an article by Dudley Ross Spears on The Mormon Plan of Salvation. There are actually scriptures in the Bible that support their ideas. The Mormon church uses the Bible and Book of Mormon together and both help to clarify and interpret each other. Two witnesses are better than one and I couldn't imagine that a just God would leave us only the Bible which has become a source of contention throughout the ages because of differing interpretations. Prophets and modern revelation are needed today just as they were needed in Bible times.
The article made it sounds that Mormon beliefs come all from the Book of Mormon or Doctrine and Covenants but the Bible is used too.
You can discredit the Book of Mormon - but it shows that you have not read all the goodness in it - have only read to pick out holes. The Bible has holes also. But they are not really holes - they are just things we don't or cannot understand but it does not make them true.
Also just a point for you to ponder - do you really think young Joseph Smith an unlearned boy was able to fabricate the Book of Mormon? He was a young, uneducated farm boy. Many learned and educated people have written essays and material for us to ponder over - but no-one would have the ability to write the kinds of things the Book of Mormon contains. The Heavenly Father has chosen the most unlikely people to change history - I am more inclined to believe that he would choose an unlearned 14 year old farm boy who was not a leader of any existing church, to translate another record of ancient people. Jesus himself was born in a lowly manner in a manger to average parents despite the fact he was the Son of God and despite the fact he would die for us.
Just my thoughts
The Bible states that it is self-sufficient and complete. See "Once for All" and "Is the Bible Sufficient?" Have you ever stopped to consider that your arguments are no different from the followers of Islam? They too claim that the Bible is insufficient and that later revelation is needed, only they claim that it is Mohammad's book and the Bible that are two witnesses.
The Bible states that prophecy would cease and that no changes were allowed to what has already been revealed. See "Latter Day Revelations."
While you claim there are holes in the Bible, like the rest you can't put your finger on any specific one. It's only a rumor you've heard and are repeating. See "What About All Those Mistakes in the Bible?"
Most non-Mormons strongly suspect that Joseph Smith didn't write the Book of Mormons, not because it was such a great work, but because he couldn't remember what he claimed to have written. Evidence shows it has been heavily edited in its short life so it is not from God either. See "He Didn't Write It" and "A History Lesson on Mormonism."