Was God's dealing only with Jews from Genesis 12 to well into Acts?
No, it was predominantly Jew only, not exclusively. From Genesis 12, the call of Abram, until well into Acts, God dealt mainly with Israel, though Gentile exceptions existed: Jonah's mission to Nineveh, Naaman the Syrian, Rahab, and Ruth the Moabitess. After that, Paul was sent to the Gentiles.
Les teaches that Romans 11 trumpets that God is not through with His covenant people Israel, even though the nation has been in spiritual blindness for about 1900 years. He explains that from Genesis chapter 12, the call of Abram, all the way until well into the book of Acts, God's dealing was predominantly Jew only. Les notes exceptions: Jonah went to Nineveh, Naaman the Syrian general, Rahab the harlot, and Ruth the Moabitess were Gentile exceptions. After well into Acts, Paul is called aside and God tells him, "I'm going to send you far hence to the Gentiles." By the end of Acts, Paul is in prison in Rome, near martyrdom, still writing prison epistles, having already been among Gentiles for many years, while going first to the Jew and synagogue, then turning to the Gentiles when rejected.