I very much doubt he had the Septuagint in mind.

Hyperion has done a study on Galatians.

Actually there's Moses too in Hebrews 9:19-20. The writer might be making a subtle point about the blood of the old covenant in saying "Moses had spoken... saying..."

The use of quotations in Hebrews is pretty unique and it links in with the opening of the epistle which simply says "God... spoke..." There is nothing like "the apostle Paul to the ecclesia in Jerusalem" and we don't even know who wrote it. That's because Hebrews is basically saying "forget any human ideas here: God already said this in the Old Testament." I.e. this is nothing new - God already spoke about the superiority of the things to do with Christ over the elements of the old covenant. And so the epistle is a series of Old Testament quotations proving this point. In chapter 1, for instance, instead of saying "as the prophet Isaiah says" it says "he said", i.e. God said it (Hebrews 1:1).

Not sure that's the point of what the writer of the Hebrews is saying. He's not concerned with that level of detail from the context. What would be the point if it was present tense? In a practical way?