Your reference to Anne's conviction that breaking off the engagement was in CW's best interest has always been my great comfort regarding her decision. However, upon this reading, I am struck (more than on previous readings) by what comes 3 paragraphs later:

but Anne, at seven-and-twenty, thought very differently from what she had been made to think at nineteen. She did not blame Lady Russell, she did not blame herself for having been guided by her; but she felt that were any young person, in similar circumstances, to apply to her for counsel, they would never receive any of such certain immediate wretchedness, such uncertain future good.

This section makes me wonder. Anne drew comfort, at age nineteen, from the thought that she'd done it out of his best interests. But was she truly convinced of this, or did she talk herself into it after having already been persuaded that her engagement was "wrong"? Anne was "guided by" Lady Russell into believing that it was in her own best interest to break off the engagement; from there, it's only a short step to believing it is in Frederick's best interests as well.