Didn't the writer of Hebrews (and others) quote the OT from the Septuagint source instead of from the Hebrew source ? I'm pretty sure even the KJV can confirm this.

Here's an example, Hebrews 10:5 says:

Heb 10:5 (KJV) Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me:

This is actually an OT quote from Ps 40:6 :

Ps 40:6. (KJV) Sacrifice and offering thou didst not desire; mine ears thou has opened.

But if you look at the Hebrew language, a word for word translation of the latter half of Ps 40:6 is closer to this: "ears you hollowed out for me." This is some sort of Hebrew idiom ... which only occurs here with both "ears" and "hollowed out" in the same sentence. So, the meaning is somewhat obscure ... those that translated from Hebrew to the Septuagint took this to mean, "you've prepared a body for me" ... and that is the way the Septuagint translated it .. which, the writer or Hebrews quotes ... but the Hebrew says "ears you hollowed out for me" ... which may mean, "you have made this very clear to me" ... which the KJV captures with the translation "mine ears thou has opened" back in the Psalms. But both ( as is often the case with prophecies ) make sense. ( For example, "Out of Egypt I have called my Son" ... referring to both Christ, or the Descendants of Israel ).

My only point I wish to make is this: your article seems to knock the Septuagint ... and yet the author of Hebrews quotes from it ... as the KJV makes apparently clear.