aqwsed12345 : In the same unfair way, you now want to reverse the burden of proof, so that ... I have to present the LXX manuscripts with Kyrios.
Was it not you (aqwsed) who said "the vast majority of surviving manuscripts, including those from the first century CE, use Kyrios" and "the widespread use of "Kyrios" in surviving LXX manuscripts, including pre-Christian ones, suggests that it was an established practice among Greek-speaking Jews". It is not unreasonable to ask you which manuscripts you have in mind when you make these claims. Perhaps you were using a bit of hyperbole and exaggerated the evidence but then I would at least expect you to own up to it, not to come up with a host of reasons that the pre-Christian LXX would contain kyrios if only we could find the manuscripts.
The manuscripts we already have are (1st Century BCE) 4Q120, P. Fouad 266, (1st Century CE) P. Oxy 3522, the Greek Minor Prophets Scroll from Nahal Hever, (1st to 2nd Century) P. Oxy 5101. You posited that if a NT manuscript from the first century was found and it contained God's name in some form, that would prove nothing. Maybe so. But if a number of fragments in different locations were found, all dating to the first century and all containing some form of God's name, that would be strong evidence for general use. Yet that is what we have for the LXX. In discussing 4Q119 and 4Q120 (both 1st century BCE), Emanuel Tov says (Textual Criticism of the Hebrew Bible, p.132) :
These are probably closer to the Old Greek than to the later uncials, which had been revised towards the Masoretic Text. 4Q119 contains a slightly freer translation than that found in the uncial manuscripts. This scroll presumably reflects the Old Greek, revised toward the Masoretic Text in the uncials. In 4Q120 Lev 3:12, 4:27, Iaw for LXX kurios probably reflects the original pre-Christian rendering of the Tetragrammaton, preceding kurios of LXX. The translation vocabulary of both Qumran scrolls was not yet standardised as it was in the later uncials.
There are two (among many) objections you raise to the idea that early Christians read and used God's name.
The first is that all existing manuscripts of what became the NT do not contain God's name. But all the manuscripts we have which might have contained God's name date to the second century and beyond, and we have already seen that kyrios had replaced God's name by that time so you would not really expect to find anything else.
The other objection you raised is that Philo does not use God's name, but that is ignoring Philo's theology. Along with the normal epithets for God, such as "eternal", "unchanging" and "imperishable", Philo produces others for which he is our earliest authority. At On Dreams 1.67, for example, God is described as "unnameable" (akatonomastos) and "unspeakable" (arrhetos), and "incomprehensible under any form", none of which terms are applied to God before Philo's time in any surviving source. So quite clearly, Philo would not use a name for God because his philosophy is that God is unnameable. I do remember reading a suggestion that possibly it was Philo himself who endorsed the replacement of God's name with kyrios, but that was years ago and I don't remember the source now. I wouldn't put it beyond him but I don't think his influence was sufficiently great.