Was it a valid offer that Israel could have had Jesus as king during His earthly ministry?
Yes, Scripture presents it as a valid offer: during His earthly ministry Israel could have received the King, and God in His providence would still have taken care of the sacrifice that had to be accomplished. God's foreknowledge knew they would crucify Him, yet their rejection was unbelief, not because the offer lacked validity.
Les teaches that, just as Israel had a valid offer of Canaan's blessings and rejected it in unbelief, the offer of the King and kingdom was also valid in Jesus' earthly ministry. If they had received Him, God in His providence would still have taken care of the sacrifice that had to be accomplished. But God in His foreknowledge knew they would crucify Him. Peter later presented the nation with the King and kingdom again after the crucifixion, and they again rejected it in unbelief. They stumbled at Nazareth and at a King coming on a donkey's colt rather than a prancing white steed, and they continued rejecting Him until God raised up the apostle Paul.