And come to think of it ... you should be arguing FOR the presence of the article! Since you believe that “he will confirm” the Mosaic covenant the presence of the article would lend support to your position. If the article was missing (as you are arguing for as the preferred reading) the better rendition would be one of unfamilarity ... i.e., “confirm a covenant” ... one that was presumably not in existence as of yet.
Nope, I go by the exact wording of the LXX. Injecting articles is a modern convention that started centuries after the spread of the manuscripts we have. That is not to say I don't agree with the addition of articles, but that it is not to be found in the manuscripts. Is it true or not that Jesus said that not one "jot" or "tittle" would not be fulfilled? Those publishing what is supposed to be the words of the manuscripts in the Greek - not the translation - and saying that those are the words written are ridiculous. The addition of Greek "articles" is not faithfully transmitting the Scriptures in the Greek language. Does that make sense?