As you pointed out, he was not using the techniques of NT textual criticism, nor did he have any other manuscripts to draw from. He was doing the work strictly based on inspiration from God. And side by side, smith's 'translation' is bogus with typical signature of one who hasn't a clue with what he is dealing with. Example - Psalm 95:11 is quoted three times in Hebrews. The JST adds nothing to the first quotation (Heb. 3:11), adds 22 words to the second quotation (4:3), and adds 5 words to the third quotation just two verses later (4:5). Apparently, smith didn’t understand that Hebrews was quoting the same Old Testament text in all three verses.
Take also this
John 4:2 — (Though Jesus himself baptized not, but his disciples,) KJV
John 4:3 — Now the Lord knew this, though he himself baptized not so many as his disciples; JST
John 4:2 — 
So if smith was so 'inspired' why didn't he translate to the more correct MS?
Smith's translation skills are bogus - he went about trying to force his doctrine upon the bible - regardless of the 'correct'. No, I trust God in the preservation of His word - and not that of a convicted treasure seeker.
It is a valid and simple question. If “god” could give Smith the entire BOM, it would seem that an “errata” sheet for the Bible would be a simple matter and would not propagate and repeat supposed same errors...
There would be no need for “as far as it is translated correctly” because “god” would have not only made the accurate BOM, a “companion” to the Bible, but would want the bible brought up to the same standard again. Unless of course “god” is just a slacker kinda guy.
I know, details, details...
However I am glad that God holds a tick higher standard.
So if smith was so 'inspired' why didn't he translate to the more correct MS? The answer to your question is found in the word more.
As for your disbelief in a prophet of God, that is nothing new. The scriptures are full of such. :-)