Jesus most certainly DID modify the law whenever it was being used to abuse the people.  Jesus healed on the Sabbath.  The Pharasees and high priests objected.  Jesus permitted his Twelve and disciples to pluck grain to eat on the Sabbath, because they were hungry.  The Pharasees and other 'righteous people' objected.  Jesus stated that the Law was created for man [and woman], not man [and woman] for the Law.

You are utilizing the old classical concepts in the wrong places.  Moral certitude is easier to attain on the level of the more general [i.e. the dignity of the human person, basic human rights], but impossible to attain with regard to specific recommendations.  With our contemporary recognition of greater diversity and pluralism, we should be even more conscious than Aquinas [who explicated Natural Law in the Summa], of the impossibility of claiming moral certitude on specific and complex issues.