You are absolutely right. The Eastern Church used and still uses the Byzantine-type text found in the Gospels of the Codex Alexandrinus, with Alexandrin-type text following in the Epistles.
The older Alexandrian-type texts, Codex Sinaiticus and Vaticanus, are not as 'conflated' ('cleaned up' or 'redacted') as the Byzantine-type Codex Alexandrinus. (I must admit I can see how the naming of these texts and Codices can lead to profound confusion).
My point was that we don't know where the Greek versions came from (I suspect most of them came from Stephanus's Greek NT), but chances are they all belonged to the redacted Majority (Byznatine-type) Text.
The fact remains that Erasmus, despite having used various Greek sources, did not have a complete NT in any one of them, with no traceable identity of any of them.
How unreliable were such sources can be surmised from the fact that someone handed Erasmus a "Greek" text containing Comma Johannenum (retro-translated from Latin to Greek), obviously a forgery, which he accepted as 'genuine.'
This forgery, then, found its way into all the western Bibles, starting with the KJV (which still carries it!). This makes a mockery of the name "word of God" for the Bible.
My other point was that the KJV, then, was most likely translated from either Erasmus's Latin version into English (most likely becauase Latin, not Greek, was the lingua franca of Europe at that time and therefore better known; we know that Luther had to use a Greek dictionary and was most likely not very proficient in Greek, thereby relying on the languge he used daily, Latin, besides German).
Thus, in KJV we have (a) questionable sources to begin with, (b) at least one doucmented forgery, which was never removed (c) doubtful translations and (e) even the known sources belong to the most 'redacted' (altered, doctored) of extant NT sources (the Byzantine-type text). And yet the KJV reigns supreme as the "word" of God!
On top of that, as my example with Isaiah 9 directly quoted from Tanach in 11,062 shows, the "Hebrew Bible" does not agree with either the Septuagint or the KJV and the two Christian sources do not agree with each other!
Also, the KJV quote from Isaiah 9 comes from somewhere other than the Greek text and certainly not from the Hebrew text as claimed by the Jewish sources.
It also must be noted that although, and even though, Erasmus used Greek texts based on the Byzantine-type text, that text, when transferred into Textus Receptus became significantly altered. It is not entirely clear who or what caused that alteration, but Erasmus is the prime suspsect, whether by mistranslation or by faulty copying.
All this points to all the reasons you seem to doubt the LXX to begin with: multiple sources, copies of copies and based on a translation. By your criteria, the entire Christian canon is doubtful. I can't say that I don't share that opinion, but the same applies to Hebrew/Jewish scriptures as well.
Those weren't Western Europeans or Protestants making all those copies of the Byzantine text and leaving the Alexandrian text type on the shelf. Those were Greek monks and scribes taking their time to make cursive copies of the Byzantine text instead of the Alexandrian.
If everyone in Eastern Europe thought that Codex Alexandrinus or the Alexandrian Sinaiticus and Vaticanus were so pure and authoritative, then why weren't they making more Greek copies of them ?? Did someone forget to notify the Greek scribes of an official change in Church policy back there, or did these monks and scribes just decide on their own to keep copying the Byzantine text and ignore church authorities???
BTW The KJV was based upon Beza's 1598 Greek Text, and though he no doubt owed much of it to Erasmus's 2nd Greek Text, dozens of Greek manuscripts had arrived in the West during those 70 years, leading to a much more accurate Greek text by that time.
It appears that as the Moslems were moving into eastern Europe, the EO monasteries were sending their valuable Greek manuscripts to the West. Thus Beza had a large collection of Greek manuscripts from which to work.