Thank you PP, for a very good Post, it is not something I have not thought about before, but it is very doubtful that the N.T writers would have consulted what would have been very old MS of the Septuagint, even in their time, and yet it is clear that the Septuagint is the version they quote from most times. ( there is evidence of other influences on their writing, in one or two places).
But, the practice was to discard old MS, use the newest, hence the older MS have been found placed in "storage" out of respect for them, but at the time of placement they were redundant.
As you say though, if YHWH were there in what they read, or perhaps they wrote it too, it would not have been an issue, as I said too in an earlier Post.
In view of your Post above, there is more doubt that it ever was there.