Any translation, such as the Septuagint (LXX), introduces the translators' biases, and the situation was very fluid, since there were several Greek translations produced by redactors and commentators over time. And the NT writers quoted from the range available to them, including some we do not have access to.
Even though Jesus (Yeshua) would have spoken in Aramaic, the Gospel writers have him citing the text from the LXX - often using Pesher interpretation (that is, a commentary, rather than a literal citation). Not that any of the NT writers ever saw Jesus or heard him speak.
The early Christians, indeed, seemed to settle on the translation by Theodotian (especially of Daniel - part of which was written in Aramaic and part in Greek - as is attested by the DSS).
The variants in the DSS provide one testimony to the fluidity of the texts.
The manner in which the Watchtower amends the text to suit its biases replicates in a small way the manner in which the Hebrews were prepared to amend the text over time.
The Hebrew text that is available today (I possess two English translations of the Tanakh) is about 1000 years old (plus and minus a century or three). It was produced by Jews known as Masoretes and they took the variant Hebrew texts, producing the current single Hebrew version. In doing so, they set previous errors in concrete - and we lost the earlier Hebrew versions.
The text of the LXX is based on a Hebrew text that was about 1500 years earlier than the Hebrew Masoretic Text (MT) - the earlier text was written in palaeo Hebrew characters - see the use of those characters with "YHWH" in the Greek DSS.
It is impossible to know what the original writers wrote (the autographs).
For an analysis of the manner in which the NT writers used the Scriptures (our "Old Testament") read: "Biblical Exegesis in the Apostolic Period" by Richard Longenecker.
Doug