The Torah bears obvious traces of the developing Jewish law. For example, between Exodus and Leviticus, “an eye for an eye” was expanded from pregnant women to everyone and apparently substituted with compensation.
They say, “If the Torah is not divine letter-for-letter, then how we know what to do?” How did the Hebrews know before Moses told them? How did the Sadducees live while rejecting Deuteronomy? We test conjectures, live normal lives, respect tradition, and assess the writings critically.
Moses judged the people well before he received the Torah. On Jethro’s urging, Moses wrote down the law and passed it to the elders. The Torah itself is clear that a significant portion of Jewish law is not divine.
People observe secular laws because they are enforceable and basically authoritative. Secular laws are promulgated because they are thought beneficial for the nation. Likewise, religious laws dating back to the priests are good enough if Israel cares to enforce them.
Theological issues need not determine our present. Strictly speaking, the Torah does not prohibit intermarriage. Rather, we refuse assimilation because everybody demands it of us; because the world tried to convert, annihilate, and expunge us; because of the stakes, pogroms, and gas chambers; because of the countless Jews burned, torn, and gassed; and because of the thousands of years we kept repeating, “The next year, in Jerusalem!”