Were God's promises to Israel still valid if He knew they would fail?
Yes, the promises and offers to Israel were still valid even though God knew the nation would fall far short. They are described as genuine valid offers. God intended Israel to become a nation of go-betweens, or priests, and a light to the Gentiles. His foreknowledge did not cancel them.
Les explains from Isaiah 49 that God had in mind Israel becoming a nation of go-betweens, or priests, and He calls these things valid offers even though He knew the nation would fall far short. The context connects this to Exodus, where Israel's obedience to the Law and the covenant given at Mount Sinai would lead, sometime down the road, to funneling salvation into the Gentile world. Isaiah 49:6 speaks of raising up the tribes of Jacob, restoring the preserved of Israel, and giving the nation of Israel as a light to the Gentiles. The validity of the offers rests in God's stated purpose, not in Israel's foreseen success.
And he said, It is a light thing that thou shouldest be my servant to raise up the tribes of Jacob, and to restore the preserved of Israel: I will also give thee for a light to the Gentiles, that thou mayest be my salvation unto the end of the earth.