aqwsed12345 : The presence of ΙΑΩ or the paleo-Hebrew Tetragrammaton in these manuscripts must be understood as variants or later revisions, not as evidence that they were the original translation practice of the LXX.
While Emanuel Tov does say that the writing of the tetragrammaton in Hebrew characters in Greek revisional texts is a relatively late phenomenon, he concurs with Stegemann and Skehan that Iaw reflects the earliest attested stage in the history of the LXX translation, when the name of God was represented by its transliteration, just like any other personal name in the LXX. Tov says (Greek Biblical Texts from the Judean Desert, pp.20,21) :
In the absence of convincing evidence in favor of any one explanation, the view of Skehan and Stegemann seems more plausible in light of the parallels provided. This argument serves as support for the view that 4Q120 reflects the Old Greek, and not a later revision/translation.
Whether, in fact, it was a revision or the earliest attested stage of the LXX translation, we know that the LXX in the first century contained God's name in one form or another.
aqwsed12345 : If the NT authors originally used the Tetragrammaton (e.g., ΙΑΩ or YHWH), we would expect to see transitional evidence—manuscripts or fragments showing a gradual replacement of the name with κύριος. However, no such transitional manuscripts exist.
It's interesting that the background to the use of kyrios as a sacred name (KY) is completely unknown to us. There is no transitional evidence - manuscripts or fragments showing a gradual use of KY - but all the copies we have of the NT contain this shortened form. There is no record of any discussion about it, or any objections to it, it just happened...universally. There are lots of theories but there is no concrete evidence of when this was introduced. It must have been in the first century, either by the original writers or by copyists. But there is not a murmur about it (that we know of). This does not require a conspiracy. It would be perfectly natural, especially after the destruction of the temple, for the largely gentile church to distance itself from Judaism. But if the original authors did use some form of God's name, the record of the nomina sacra (specifically KY) shows that any transitional evidence has been lost.