You write that “the earliest copies of the LXX used various forms of YHWH” though you nowagree that the earliest example in the Rylands fragment does not.
What I am saying (as the scholars have said, and the photograph shows) is that PRylands 458 does not contain either YHWH or KYRIOS because the manuscript is not extant in that part of the text where the divine name appears.
Jewish fragments of the LXX can be divided into three groups with respect to the divine name:
1) fragments of the LXX that do not preserve parts of the text with the divine name.
2) fragments of the LXX that do preserve examples of the divine name in various forms.
3) fragments of the LXX that preserve KYRIOS in place of the divine name.
There are many fragments that fall into category 1) and PRylands 458 is one of those. There are around 7 fragments that fall into category 2) including the famous Fouad 266, the Minor Prophets scroll(s) and others. There are no fragments that fall into category 3) because there are no Jewish copies of the LXX that substitute KYRIOS for the divine name.
I don't know what more to say about your claim that Christian copies of the LXX don't exist. It's not how scholars present the data, including Jewish scholars such as Emanuel Tov and Robert Kraft. For example Robert Kraft's whole website is based on the notion of comparing Jewish and Christian scribal practices in their copies of the LXX. If you were correct that there are no Christian copies of the LXX then his whole project would not make sense. Or the many textbooks that discuss Christian copies of the LXX.
It is true that Jews translated and circulated the LXX. But the text as it comes down to us was preserved through Christian scribes. The modern text of the LXX and translations of it are based on the Christian copies of the fourth century and later. There are no complete Jewish copies of the LXX extant. There are only fragments, such as PRylands 458 and others we have been discussing.