I think Joseph Smith believed in the Book of Mormon he was reading from. He really believed what he had written and preached to people. That doesn’t mean that it is true. It means he believed it was true. Likewise Hyrum believed in his brother.
Lots of people believe in a lot of different things. They truly do. That doesn’t make thier beliefs true.
Have you considered the actual story of the coming forth of the Book of Mormon? First we have the story of the First Vision:
Joseph Smith–History:
15 After I had retired to the place where I had previously designed to go, having looked around me, and finding myself alone, I kneeled down and began to offer up the desires of my heart to God. I had scarcely done so, when immediately I was seized upon by some power which entirely overcame me, and had such an astonishing influence over me as to bind my tongue so that I could not speak. Thick darkness gathered around me, and it seemed to me for a time as if I were doomed to sudden destruction.
16 But, exerting all my powers to call upon God to deliver me out of the power of this enemy which had seized upon me, and at the very moment when I was ready to sink into despair and abandon myself to destruction—not to an imaginary ruin, but to the power of some actual being from the unseen world, who had such marvelous power as I had never before felt in any being—just at this moment of great alarm, I saw a pillar of light exactly over my head, above the brightness of the sun, which descended gradually until it fell upon me.
17 It no sooner appeared than I found myself delivered from the enemy which held me bound. When the light rested upon me I saw two Personages, whose brightness and glory defy all description, standing above me in the air. One of them spake unto me, calling me by name and said, pointing to the other—This is My Beloved Son. Hear Him!
Then we have the story of the visitation of the Angel Moroni:
Joseph Smith–History:
30 While I was thus in the act of calling upon God, I discovered a light appearing in my room, which continued to increase until the room was lighter than at noonday, when immediately a personage appeared at my bedside, standing in the air, for his feet did not touch the floor.
31 He had on a loose robe of most exquisite whiteness. It was a whiteness beyond anything earthly I had ever seen; nor do I believe that any earthly thing could be made to appear so exceedingly white and brilliant. His hands were naked, and his arms also, a little above the wrist; so, also, were his feet naked, as were his legs, a little above the ankles. His head and neck were also bare. I could discover that he had no other clothing on but this robe, as it was open, so that I could see into his bosom.
32 Not only was his robe exceedingly white, but his whole person was glorious beyond description, and his countenance truly like lightning. The room was exceedingly light, but not so very bright as immediately around his person. When I first looked upon him, I was afraid; but the fear soon left me.
33 He called me by name, and said unto me that he was a messenger sent from the presence of God to me, and that his name was Moroni; that God had a work for me to do; and that my name should be had for good and evil among all nations, kindreds, and tongues, or that it should be both good and evil spoken of among all people.
Then we have the intricate history of him obtaining the plated; the persecution that followed by people who tried to steal them from him; the loss of the 116 pages of the original translation; the process of translating the plates by the Urim and Thummim (and partially by the seer-stone); the involvement of several people who wrote for him as he dictated the text; the testimony of the eleven witness; and many other factors involved that would be too lengthy to relate in any detail. Now if he just “believed” all of that was true when in fact it wasn’t, that would have made him (and a lot of those involved with him) the most deranged, hallucinating, psychotic nutcases ever! You can accuse Joseph Smith of anything you like; but I don’t think you can accuse him (and those working with him) of having been utterly mad! That is the one accusation that I don’t think fits the bill.