Here "in his heart" refers to his deepest inner being. AT: "He treasures the commands of his God in his inner being" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here failure to obey Yahweh is spoken of as slipping off a safe path and falling. AT: "he will walk safely in the way God wants him to walk" or "he will safely do the things God wants him to do" (See: [[rc://en/ta/man/translate/figs-metaphor]])