What is the error?
I don’t think you are understanding how Catholics view scripture. A probable dual authorship does not negate that what is being taught is inspired. We understand scripture comes to us via human hands, human means in a human context.
The claim for the Book of Mormon is that is came to be via supernatural means, both in how Smith acquired it and how it was translated. The translation part, in particular, should be perfect. When substantial changes are made to it, how would you know that the changes are right? You have NO documents to go back to in order to verify the translation.
It is a poor claim to say we don’t have old documents for the Bible. You should study up on how modern translations of the Bible are being done. The methods, and available documents to translate from, compared one against the other, are far improved over the days of the King’s men.
I also think that you should stop giving the impression that Mormonism teaches the Bible and Book of Mormon are equally reliable, when it is clearly Mormon teaching that the Bible can be trusted only so far,while the Book of Mormon can be trusted with absolute certainty.
Like most things, Mormons have everything turned upside down.