About the marriage and divorce issue in Matthew 19:9, Jesus was replying to the Jewish people. Does this mean that it does not apply to Christians but only Jews?
You are missing an important implication. When laws were given to people there is an immediate audience and implied general audience. For instance, when the Ten Commandments were given they were given to the Israelites who left Egypt. "The LORD our God made a covenant with us at Horeb. The LORD did not make this covenant with our fathers, but with us, with all those of us alive here today" (Deuteronomy 5:2-3). However, those commands were not limited to that generation only. "Now this is the commandment, the statutes and the judgments which the LORD your God has commanded me to teach you, that you might do them in the land where you are going over to possess it, so that you and your son and your grandson might fear the LORD your God, to keep all His statutes and His commandments which I command you, all the days of your life, and that your days may be prolonged" (Deuteronomy 6:1-2).
The same is true of the New Testament. Most of the teachings of Jesus were to a Jewish audience, but they were to be passed on. "And Jesus came up and spoke to them, saying, 'All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age'" (Matthew 28:18-20). Thus, while Paul wrote to the Corinthians about the problems in their congregation, those teachings still apply to us.
This line of argument has been around for many years as an attempt to avoid applying Jesus' explanation of allowable divorce to be applied today. However, the approach leads the advocate to claim that all four of the gospel accounts do not apply to Christians, which is logically false. See