This is an amazing text. Clearly Paul of all people is persuaded that there is nothing lacking in the sacrifice or the sufferings of Christ. He also knows that, of himself, he cannot produce the sinless nature that you describe, rightly, as necessary to pay the debt of atonement.
So how can we take the text seriously in the light of Paul's other teaching?
I think the key is Paul's “Now I live, yet not I, but Christ lives in me.”
Do you see where I'm headed?
I've looked this over in the Greek and the RSV as you asked and maybe I'm just not getting what you're driving at here. What has this got to do with Mary's status as Co-Redemptrix?