You have to admit that this is an extremely subjective means of determining the content of Scripture. Considering how easy it is for humans to delude themselves - knowing about the Fall of Adam and Eve, do you think God is going to rely on the "Spirit" enlightening every individual Mormon? For a book to be considered God's Word, we have to accept that historical witness that they give. Compare the Apostles and Joseph Smith - their respective histories and their personal virtues. Considering we see no evidence of a huge battle that took place in upstate NY, or any Golden plates, or any evidence of an ancient civilization that would prove the Book of Mormon to be true, we can say that the whole thing is pretty much a hoax. There is no historical proff that any of it existed - which casts great doubt on the whole affair. Compare this to the Apostles of Christ.
Regards
No, I believe that God will enlighten anyone—not just Mormons—who asks him in faith. That is what Jesus taught:
Ask, and it shall be given you; seek and ye shall find; knock, and it shall be opened unto you: For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened. (Matthew 7:7–8)
Is personal revelation from the Holy Spirit subjective? Perhaps. But so are faith, hope, and charity, which are essential to a Christian life.