How could Israel freely reject God's offer if God knew they would reject it?
God's knowing beforehand did not remove Israel's free will. He made a valid offer while knowing Israel would not accept it, but Israel did not know that outcome. They acted voluntarily, as people and nations do, and their rejection brought about the crucifixion required for salvation.
Les teaches that God made a valid offer to Israel, and although God knew Israel would not accept it, Israel did not know that. Israel still acted of their own free will, just as humankind and nations do today. Israel responded to the offer by rejecting it, and by rejecting it brought about what had to happen for our salvation: the crucifixion. Jew and Gentile together had to be part and parcel of His crucifixion.