It seems odd that you stop quoting the blog at an important point:
(Brief detour: Some may want to play around with the idea that Paul was quoting from the Vorlage that the translator of Greek Isaiah was using. Possible? Sure. Probable? Not really. First, the translator of Old Greek Isaiah is known for his free translation technique — interpretative moves in the Greek text are nothing new. Second, we have no textual evidence for a different Hebrew text here.)
So here the author pulls in two lines of thought:
1. The Greek was known for being a "free translation technique," that is not sticking closely to the Hebrew original.
2. It is important when there is no textual evidence of a Hebrew original matching the free translation of the Greek.
Now you take those points and apply them to the LXX translation in the OP, and you are left with a free translation with no Hebrew original verifying it. When they met a difficult word or syntax they had to handle it in some way. And you can see the mess they made of Isaiah 9:6:
For a child is born to us, and a son is given to us, whose government is upon his shoulder: and his name is called the Messenger of great counsel: for I will bring peace upon the princes, and health to him. (Isa. 9:6 LXE)
Does this strike you as a trustworthy translation of the beautiful Messianic description we've come to know and love? Is this what you profess to be your original inspired text, denouncing the corrupt Hebrew that calls him Mighty God and Father of Everlasting?
We must be cautious with the LXX and use it knowing it is not perfect.