Joseph Smith wasn’t given the divine BOM in private. There were 11 other witnesses.1. If Mohammed was given the allegedly divine Quran in private by Gabriel, and Joseph was given the allegedly divine BOM in private by Moroni, why should we reject one and not the other?
2. Given that 1) both Islam and LDS defer to their post-Biblical documents as superior and 2) count inconsistencies of the Bible with their post-Biblical documents as corruptions in the Biblical text, why should the BOM be accepted while the Quran is rejected?
As I understand the Koran, it rejects the entire gospel message that the Apostles gave claiming that they were mistaken about the death and resurrection as well as the divine nature and sonship of Christ. The Book of Mormon on the other hand emphasizes and testifies of the truthfulness of the apostles message. Islam teaches that the New testament was changed while the Book of Mormon teaches that important doctrines were left out. There is a big difference between the two. We don’t doubt the truthfulness of what is in the current Biblical record, we simply don’t believe that everything important that Jesus or his apostles had to say was passed down See 1 Nephi 13.29
You claim that to deal with supposed inconsistencies, mormons simply say the BOM must be correct and the Bible mistaken. I am struggling to think of situations where this is the case. Christians make up inconsistencies between LDS doctrines and the bible just like atheists make up inconsistencies between different bible passages but there are very few, if any, true contradictions.
For example, you bring up 2 Timothy 3:16-17 as if it is a contradiction. Note however that it says “all scripture” not, the old Testament and New testament (which as you properly state wasn’t yet completed.) I am still uncertain as to where you get the idea that the canon of scripture was ever meant to close.
As far as the Melchizedek question goes, the first 7 non LDS commentaries I found after a short google search of Hebrews 7:3 state that this verse doesn't mean that Melchizedek had no mortal beginning or end but that this verse in Timothy refers to the fact that his priesthood didn’t come down through a genealogical lineage of priestly fathers like the Jews were accustomed to but directly from God as a stand alone priesthood, just like Jesus’ priesthood did. See the commentaries here. http://bible.cc/hebrews/7-3.htm
This of course is exactly what Alma chapter 13 is pointing out, namely that he did have a father, but that isn’t where his priesthood came from. There is no contradiction here, only in that a misunderstanding of the Jewish Idioms can cause confusion. Of course there is also a chance that these 7 commentaries are all wrong, and Alma is also.
I don’t accept him over the apostles, I accept him with the apostles, just as I accept Paul with the other apostles not over them. Joseph was an apostle also chosen of God. I accept him with the others.3. How can you accept Joseph Smith over the apostles?
I don’t believe the foundation of the church was ever shaky, I do believe that the falling away prophesied by Paul has already occurred and that during this falling away the foundation of the church had no mortal representatives for a season. That season ended with the restoration of the gospel and the Lord has chosen mortal men to fill the role of apostles and prophets once again in preparation for the physical return of the chief cornerstone.
That is how I understand the answers to your questions anyway but I am always open to adjusting my views upon receiving further light and understanding.