You objected to the Christian bona-fides of Mormonism on the basis of adding other extra-Biblical books. That is not a valid criteria.
Via the Cairo Genzia. After the fall of Jerusalem in 70 A.D., the Jewish texts underwent constant charge in response to Christianity at the hands of copyists, until there was a Christian demand for a pure text, so they manufactured a new fixed text for them, claiming to be from "old manuscripts saved from the Temple of Jerusalem", the Masora (Traditional) texts became the new standardized canon, and it disagreed with the Septuagint in several places, and so the Septuagint "was declared to be the work of Satan" (Kahle, The Cairo Genzia 138-139). Every Jewish copy of the Septuagint or Greek scriptures was being systematically destroyed, while they toiled to fix the Hebrew text for the Christians (Kahle, The Cairo Genzia 86, 108, 118, 127). Jews forbid the reading of "the outside books" (Mishnah, m. Sanhedrin 10:1) and they were all burned (Talmud, b. Gitten 45b). They hid, burned and replaced any and all documents that seemed to support the ancient origins of the ascending Christians, whom they hated above all other Gentile pagans (Origen, Psalms Homily 1).
Today we know that the Old Testament was manipulated by the Jewish Masoretes, and in the 6th century Jacob ben Chiyim, "he was convinced there was only one correct Masora - the Masora compiled by himself and that the text which had been established by the great Masoretic authorities of Tiberius" (Kahle, The Cairo Geniza 71, 77; 2nd Ed 130). After they thought they had destroyed all older variants of the Hebrew texts and established their new one as an original. The Masoretic texts went unchanged for years, and this was taken as proof of being an original, but only because we didn't know they successfully suppressed and destroyed all others. It became accepted as "the only authoritative text up to the present day" (Kahle, The Cairo Genzia 71-72; 2 Ed 131) The Romans Catholics also began to destroy and archive away all of their variants and claimed their texts as the oldest ones (Kahle, The Cairo Geniza 85).
Even when there appeared discoveries of older manuscripts proving Jacob ben Chiyim was wrong, the scholars still wouldn't accept them, because they already possessed a preestablished system they 'know' is correct, so they'd correct the manuscripts instead, believing they needed to control and conform these discoveries to their own. Academia is a conservative world, it doesn't like things that makes their life's work appear to be the useless guess work it really is. Even when scholars accept a new discovery, they play it off to save face and make it appear it is as to be expected; hence the only thing they say about what the Dead Sea Scrolls show is that it proves the Bible is 'mostly' translated correctly. Do they bother to tell you about what things are not correct?
Despite that the Christians were accusing the Jews of altering the texts to hide Christian proof texts, Jerome operated under the belief that the Jews possessed a singular original text, even when though there were parts that were very significant to the Christians missing from it. All so they can reach a consensus with the Jews on what is recognized as scripture. They were played. The Bible is not, as we supposed, accounts preserved by dutiful scribes.