Again, the divine name was NOT used in the Septuagint. There is a transcription of what MAY be the divine name in Paleo-Hebrew, that made it into SOME translations, a language contemporary Jews would not know how to read or pronounce. The fact it was not transliterated into Greek unlike EVERY other name in the scripture points that there was something mystical or unknown or forbidden about the name and its use.
The use of this Paleo-Hebrew string of characters into translations is not consistent over the ages, people without education into the Jewish belief system wouldn’t know why the characters were there, hence it did not get taken over into every translation and it was often replaced with what the translator thought it meant (El or Elohim from the Canaanite god “Yahweh” was built upon, Lord, God etc) based upon the person or family the translation was intended for.
There is no conspiracy theory, the original name and its pronouncement, if it ever existed, was adapted over time and lost to history. Some contemporary references claim that only the High Priest had access to this secret knowledge but even that isn’t well founded. You’re claiming that for 1500+ years the “name” survived in general parlance and then was completely eradicated in 200 years on behalf of a small sect of Jews (Christians) because one of their factions (Catholics) had a conspiracy to introduce a trinity 100 years later.
The Jews today, perhaps even the Muslims would have some surviving references over the years like they do with everything else, the Talmud and Quran was written around the time you indicate this change would’ve happened in Christianity, yet neither of those also do not pronounce the name today and do not make claim to knowledge about the name or its disappearance at that time and throughout time the Talmud consistently translates the Tetragrammaton as Lord or God and consistently tells the Jews not to try and pronounce the name and it consistently condemns those who tried to.