The Greek translation of the Old Testament came about during the period between the Old and New Testament; but it is not the original language of the Old Testament and is itself a translation of the Hebrew. Don’t forget that.
I'm going to deal with some of the false claims "pasted" in this quote. I have no idea what is actually your commentary and what is from someone else. I'm not going to even try to figure it out in your full response.
The Hebrew that survives today IS IN NO WAY exactly like the words that came from Isaiah. In fact, many linguistic critics of Isaiah from the DSS believe the manuscripts comes from two different writers/prophets.
Isaiah lived in the 8th century BCE. The DSS is from no earlier than the 1st century BCE.
That is 7 centuries removed from the source. Much happened in that time.
Much of this is new to you. You haven't lived with the knowledge you're gaining for very long. Don't act as if you've really come to terms with the vast amount of information needs to be addressed.
Isa 40 is where most of these problems arise with those who claim two or three authors wrote Isaiah.
Isa 40:3 is a very important passage that demands the LXX stream is authentic to one author.
Isa 40:3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make straight the paths of our God.
Mat 3:1 In those days came John the Baptist, preaching in the wilderness of Judaea,
Mat 3:2 And saying, Repent ye: for the kingdom of heaven is at hand.
Mat 3:3 For this is he that was spoken of by the prophet Esaias, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
Joh 1:23 He said, I am the voice of one crying in the wilderness, Make straight the way of the Lord, as said the prophet Esaias.
Pay attention to Mark....
Mar 1:2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
Mar 1:3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
There is a conflation here that takes place with the surviving "Hebrew" .... update later so I can respond to another of your posts.