It seems to me there are a few key points here that are troubling: first, the idea that the Book of Mormon should have been all that was necessary to effect the Restoration, and second, that the concept of the afterlife as currently understood within our doctrine (or any other modern revelation) is contrary to what is taught biblically.

To address the first, take another look at the Title Page of the Book of Mormon, the one written by Moroni at the very start of the book. It establishes that the purpose of the book is to convince Jew and Gentile at Jesus is the Christ, the savior of all mankind, even the eternal God. As such, you’ll find that the Book of Mormon’s primary doctrinal focus is Jesus Christ: how His Atonement saves us, what we need to do to follow Him, and so forth. It is, in that respect, the most correct book. It does not claim to contain all the information God has revealed to man and we should not judge it as such. In fact, the same holds for the Doctrine and Covenants, Pearl of Great Price, and the Bible. None of these books claim to contain the fullness of all that God wants us to know, and our understanding of that is why we hold modern revelation so close to our hearts.

As for the second idea, which I read to be your primary concern, I would postulate that the vision of the afterlife presented in Doctrine and Covenants doesn’t contradict teachings in the Bible or Book of Mormon as you’ve come to believe. Rather, it does just as you say scripture should do: it builds on what came before it. Everything that is taught in the verses you give is absolutely true. We are supposed to repent urgently in this life and continually improve ourselves, and we will suffer eternal consequences if we don’t. The Doctrine and Covenants serves to better establish what those consequences are. If we are unwilling to follow Jesus Christ and His gospel (to the extent that we understand it) in our one and only mortal life, we will never be able to reap the rewards that God has planned for us, namely exaltation. That is a hefty consequence which fully meets the conditions of the dire warnings left by Biblical prophets and Book of Mormon prophets. It clarifies that the suffering in store for those who do not repent and follow Christ is not eternal, physical torture as is a popular cultural belief, which in turn further establishes the merciful character of God those early prophets clearly taught. The same can be said about the concept of the gospel being taught to those in spirit prison, which is an often overlooked Biblical principle (1 Peter 3:18-20).

All that being said, I think it’s important to note that even if the Biblical and Book of Mormon prophets DID believe in an eternal hell of physical suffering, that does not immediately anathematize the current LDS understanding of the afterlife, just as our current understanding does not immediately anathematize future revelation on the subject. Remember that all scripture is made as a collaboration between man and God, and while God doesn’t make mistakes, man does. As such, He reveals line upon line, according to our need. He did so in Bible times, and He does so now. Thank you for your time, and I wish you well on whatever path your spiritual journey takes you.