Here "the promise may rest on grace" represents God giving what he promised because of his grace. AT: "so that what he promised might be a free gift" or "so that his promise would be because of his grace" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here the word "us" refers to Paul and includes all Jewish and non-Jewish believers in Christ. Abraham is the physical ancestor of the Jewish people, but he is also the spiritual father of those who have faith. (See: [[rc://en/ta/man/translate/figs-inclusive]])