Interesting that you hold your point on the Jews "corrupting" the translation of the Hebrew text, but cast derision when we state believing in the Bible,"so far as it is translated correctly."
You are referencing only a few hundred years in that case, when the difference between the LXX and the prophecies of Joseph of Egypt is almost 2,000 years. In that time the scriptures were written, lost, rediscovered by Josiah's servant Hilkiah, disputed among the Kingdom of Israel and the Kingdom of Judah, appendaged, and restructured after the deaths of prophets (like Isaiah) who in life were disputed by religious leaders. There are at least twelve lost books, which are referenced but no longer available to us in the Old testament and even two epistles and possibly an additional referenced but missing from the New Testament. The LXX is hardly an undefiled canon of scripture.
You would have us believe that thousands of years of testimony by men who walked with God, from Adam to Noah, living from 300 to 900 years each, is to be found in a mere eight chapter narrative in the Book of Genesis? All that God found relevant to reveal to them is contained in 11 pages of a 1590 page record? That assertion is what defies belief!
People lived for 900 years but that other assertion is what you found that defies belief?
The Septuagint is not 'infallible' ... that is not the lesson here, but it does aid in correcting texts obviously corrupted since the Lord quoted the Septuagint. Your cult is all about sowing doubt into which the lies of the author of the BM desires to spew his heresies. It is Mormonism which began with the assertion that all Christianity was in apostasy since the death of the last Apostle, a fact most inconvenient to your cult and which it now seeks to dissemble by any means available, but mostly through a persistent assault on truth of the nature of your false prophet and his followers who 'established' your 'extra-Biblical scriptures'. And I would remind you of your cult's claims for the extra verses (hundreds) Joseph Smith added while supposedly 'translating' the King James Bible afresh ... without any data of older manuscripts before him, merely his imagination. No wonder your cult leaders haven't the guts to use as your fundamental text the Joseph Smith 'translation' of the King James Bible!