Everything stands or falls on the Book of Mormon.
Likewise, everything stands or falls on the [1820]First Vision.

The Book of Mormon does not teach that God and Jesus were seperate beings. It teaches that they are the same. The BoM teaches that Jesus is the very Father.

In 1831 Joseph Smith translated Genesis which became the Book of Moses as a part of the JST which portrays God and Jesus as the same being. In todays LDS Bible Luke 10:22 the footnote refers to the JST (written in 1831) again indicating that Jesus is the very Father. In fact, he made several changes that clearly indicate the Book of Mormon view of God.

When the 1835 Doctrine and Covenants was printed, it included the Lectures on Faith which essentially mirrored the Book of Mormon idea of God. The current Doctrine and Covenants prior to section 130 also holds this view.

So-
If Joseph Smith had had a vision of God and Jesus standing side by side as physical beings in 1820, he made no effort to correct the Book of Mormon, and he proceeded to deny that vision by incorporating the Book of Mormon view into subsequent scriptures. The Mormon church says that scripture is what is written by the prophet when moved upon by the Holy Ghost. It also declares that the Holy Ghosts prime mission is to testify of truth, and of Jesus Christ.

If the Holy Ghost witnessed to Joseph Smith during those formative years when the scriptures were produced, then it stands to reason that in later years Joseph (being guilty of adultery, polygamy and of other crimes) had lost the spirit when he proclaimed his famous King Follet Discouse.

This discourse was also given only a couple years after the first tellings of the official First Vision story.

And because the church subscribes to Josephs later speculation as revealed in the King Follet Discourse, then the church has fallen from the truth as well.

Only by dismissing the Book of Mormon, The Book of Moses, The sections of the Doctrine and Covenants prior to 130, as well as the JST, can the church justifiably adhere to its present theology.

But by doing so, it is necessarily false.

Only by admitting that the church was wrong to follow the KFD, can the church claim that it is true according to the scriptures, but by doing so reveals that the LDS church is again false.

... can it be that Law and others were right and Joseph was a fallen prophet? I'm not saying I believe this, just ... well, how do knock down that argument?

My answer: if the book of mormon is made up, then he wasn't a prophet to begin with.

The Mormon Delusion Vol. 2 by Jim Whitefield. He shows where Joe first went with God, Jesus, and HG as one entity, and then later his ideas evolved into them being 3 seperate beings.

The various scriptures and revelations were changed as his ideas changed, but there are enough that were not changed to show he was full of crap.

"Jesus was the creator under the guidance of the father so he is the father of the world/universe blah blah blah so the BoM is right still."

TBMs are experts at mental gymnastics. So much so that I can still do them almost instantly from a TBM perspective on what is otherwise a rock solid argument. To anyone that isn't brainwashed that is.