The best support of the Septuagint over other manuscripts neglects textual criticism that's a standard approach. The following is from
https://creation.com/lxx-mt-response
In two recent papers published in creationist literature, Henry Smith has argued that the Septuagint preserves a better chronology in Genesis 5 and 11. However, we believe his scholarship is significantly flawed. But we hope every reader understands that this is not personal. It is only grudgingly that we are brought to this point. Smith is a friend who has contributed a widely cited paper to our Journal of Creation. Both papers are freely available online. The first is titled: Methuselah’s begetting age in Genesis 5:25 and the primeval chronology of the Septuagint: a closer look at the textual and historical evidence.1 The second is titled: The case for the Septuagint’s chronology in Genesis 5 and 11.2. Due to the sheer number, quotes from these papers will be parenthetically cited as “ARJ” or “ICC” along with a page or note number.
Smith did not have to address our prior work, but in ICC (p. 120) he systematically misinterprets our main arguments before concluding, “Cosner and Carter deduce that the MT’s chronology is original, a conclusion that was baked into the methodological cake from the outset.” (ICC p. 120). This shows that he does not know how we came to our parsimonious text, which we have refined and will hopefully be presenting an updated version of soon. There were a few weaknesses in our first work, which he could have pointed out. Instead, he issued a general denouncement without showing that he actually understood the methodology. Yet, there are weaknesses which pervade both of Smith’s papers. First is the oddly stilted and selective nature of the bibliography. Why are there so many obscure chronographers while the best MT supporters are not even mentioned, or quickly dismissed? Why does Smith not note the multiplicity of opinions regarding which text is superior and why changes might have been made? Why does Smith cite supporting witnesses from the 12th century, but not Bede, an important opposing witness from the turn of the 8th? Simply put, his bibliography does not look like he started out with an unbiased search for the truth. There are far too many secondary sources, and there are far too many obscure, hard-to-find authors from only several centuries ago. One cannot trace many of his claims back to primary sources.
When examining Smith’s work, a problem with his sources comes up (the introductory paragraph in the ARJ paper). Concerning the date differences between the MT and LXX, he says “Eusebius (AD 260–340) is the first known author to explicitly cite and discuss the divergences, followed by Ephraem of Syria (AD 306–373)…” (ARJ, p. 169). But in the note for that statement, he says,
“Ephraem of Syria is the first known ancient source to explicitly argue that the Jewish rabbis of the second century AD deflated the primeval chronology by ca. 1300 years in their Hebrew MSS for the purpose of discrediting Jesus as the Christ: ‘The Jews have subtracted 600 years [in Genesis 5] from the generations of Adam, Seth, etc., in order that their own books might not convict them concerning the coming of CHRIST: he having been predicted to appear for the deliverance of mankind after 5500 years.’ Cited in: Hales (1830, 278). For additional citations of Ephraem’s claims, see: Assemani (1719), Wacholder (1974, 99), and Anstey (1913, 46).” (ARJ, note 3, all punctuation and italics in original).
But Smith did not cite an original source for the Ephraem quote. As good students of biblical scholarship, we should trace everything back to the original if possible. But after repeated attempts, we were not able to authenticate this quote by Ephraem the Syrian, nor to trace it further back than Hales (who Smith referenced in the quote above). This is a critical piece of evidence for those that support the LXX’s authority. Smith repeats claims of some ancient authors that the Jews deflated the primeval chronology. There is no traceable evidence of a gathering of Jewish leaders that would have the ability or the authority to do this. The burden of proof is on Smith to document when and where the Jews decided to take this route, and how they implemented it. Smith also cites the Bibliotheca Orientalis by J.S. Assemani for the ‘deflation’ claim, which would take it back about a century (to the 1700s). I (LC) tried to find this quote in the massive four-volume work, which is written entirely in foreign languages, but given that Smith did not give a clue as to where the quote was contained therein, I was unable to authenticate it. I was also unable to get a copy of Wacholder. While Anstey also references the quote, he does not give an original source. Furthermore, Anstey clearly asserts that Ephraem was wrong in his claim! In any case, the quote by Ephraem is not admissible if it is not able to be traced back to an original source. And the burden of documentation proof is on the one who would admit it as evidence.
Yet, Ephraem the Syrian wrote a commentary on Genesis. One might expect to be able to ascertain his specific interest in chronology, and what text he was drawing from, by consulting that work. In his comment on Genesis 5, Ephraem says,
“Then after he [Moses] had finished writing about the tribes of the descendants of Cain and had completed the story of the words of Lamech to his wives, [Moses] turned to record the generations of the house of Seth, beginning from Adam, saying that when Adam had lived one hundred thirty years, he begot a son in his own likeness according to his image.”3
But the LXX has “230” years for Adam’s age when Seth was born. Ephraem also says that Noah was 500 when he bore his sons, about which all traditions agree.
A statement in Ephraem’s commentary conclusively shows he was using a MT-like chronology. Regarding Melchizedek, he says:
“This Melchizedek is Shem … Shem lived not only to the time of Abraham, as Scripture says, but even to [the time of] Jacob and Esau, the grandsons of Abraham.”4
This statement is true if we use the MT chronology, but is impossible with an LXX-like chronology, for with the extended ages of paternity in the LXX chronology of Genesis 11, Shem would have been long since dead. How can anyone claim that Ephraem viewed the MT chronology as a corruption by unbelieving Jews, when he was happy to use it in multiple places in his Genesis commentary?
Smith’s quote above claims to be from Ephraem the Syrian, but it is not yet traceable back to anything Ephraem the Syrian wrote. We could not authenticate it and Smith provides no way for anyone else to effectively do so. Furthermore, Ephraem’s extant work includes a commentary on Genesis in which he uses the Masoretic text!
A helpful reader was able to provide us with the Wacholder reference. This allowed us to pinpoint the specific location of the quote within Assemani's Biblioteca Orientalis. Assemani traces the quote to Ephraem’s Exposition of Genesis and Exodus, which as we showed uses an MT chronology and lacks any such statement by Ephraem.
We are unable to trace the quote further than Assemani. In any case, it is clear that an entire line of scholars, from the 18th through the early 20th centuries, transmitted an apparently fake quote. This has been cited by numerous modern LXX advocates who also did not bother to authenticate it. We have attempted exactly that, and not only were we unable to do so, but we found strong evidence that Ephraem believed no such thing.
The situation doesn’t get any better when we try to verify what Smith said in the second half of this one sentence. Here it is in full:
“Eusebius (AD 206–340) is the first known author to explicitly cite and discuss the divergences, followed by Ephraem of Syria, Jerome, (AD 340–420), Julian of Toledo (AD 642–690), Jacob of Edessa (AD 640–708), Byzantine chronology George Syncellus (d. AD 810), and Armenian annalist Bar Hebraeus (AD 1226–1286), just to make a few” (ARJ, pp. 169, 171).
Following that, he claims:
“Each of these ancient authors (save Jerome) argued that the Jewish rabbis in the second century AD deflated the primeval chronology by ca. 1300 years in their Hebrew manuscripts to discredit Jesus as Messiah” (ARJ, note 4).
We have examined Ephraem of Syria and found no such thing. Eusebius argues for the superiority of the LXX, but as Smith admits in the very same note, “Eusebius does not attribute the motive to messianic chronology and discrediting Jesus, [sic] rather, their purpose was to encourage their contemporaries to lower their age of marrying” (ARJ, note 4).
The Jewish Encyclopedia claims Julian of Toledo accused the Jews of falsifying their chronology in connection with an argument that the Messiah was to arrive in the “sixth age”.5
We had to search for this information, however, because Smith did not provide this source as a reference. Rather, his source says that the Jews were basing their eschatological interpretation on the Babylonian Talmud. It also claims that Julian of Toledo’s refutation:
“ … interprets the texts of the New Testament that explicitly point to Jesus Christ as the Messiah foretold by the prophets of the ancient covenant, whose texts are interpreted by the apostolic writers as indicating the fullness of time, and not following the calculation of years. Jesus, the Messiah, was in fact born precisely in the fullness of times. It is, therefore, not the calculation of years that counts, as the Jewish scholars maintain, but rather the whole of the historical facts in which Jesus presents himself as the true Messiah of God.”6
This LXX-supporting witness defines the “sixth age” by epochs demarcated by events in salvation history, not by calendar dates. This is the same argument that Bede (who was clearly a MT supporter) used in his Letter to Plegwin.7