In any case, this thread was about Lewis. Does the myth argument seem valid to you?
I realize that I'm arriving a little bit late to the party. I had never visited this particular sub-forum until yesterday. I will try to answer your original question directly. First just let me express my appreciation for the contributions you've made here and for your dedication to remaining polite even when things get heated. If we had more peoplpe like you, the board and the planet would be a much nicer place.
Personally I love C. S. Lewis. I find his fiction brilliant and his non-fiction even more brilliant. As for the particular subject of this thread, I agree with it in outline, though it's probably not the first thing I'd bring up if I were debating the divinity of Jesus. The thing is, if I recall correctly, Lewis was a chaplain during WWII. He gave sermons to men who would shortly be fighting the Germans and who might be dead in a few days or hours. In those circumstances he needed an argument that was short and clear, not a treatise that covered every possible detail. However I have found the basic argument to stand up to scrutiny.
If I were arguing it, I would begin with two facts:
Fact 1: Jesus made changes to the Laws of Moses.
Fact 2: Jesus forgave sins.
Now why do I classify these as facts? Because every source that we have about Jesus agrees on these facts. All four gospels agree, the epistles agree, the early martyrs and church fathers whose writing we have agree, and the best historical analysis that we have agrees. While I acknowledge the possibility that some stories, parables, and quotes attributed to Jesus may have been made up by his followers or copied from other sources, I think these two facts are so central to everything we know about Jesus that they can't be entirely false. Moreover they not only appear in all our sources, but they appear frequently in all our sources, and if they were removed the entire narrative would be senseless. It was Jesus' willingness to change laws and forgive sins that set up the main conflict with the authorities that is the centerpiece of the gospel story.
So why are these two facts so important? To answer that, we need to understand the difference between how law and sin are understood in our time vs. that time. Today laws exist and they're important to us. However, we acknowledge them as a human creation. In the USA our highest law is the Constitution and there are various levels below that. Other countries have other systems. Now if we met someone who says "murder should be legal" or "the speed limit should be lowered to 20 miles per hour", we might find those beliefs a little odd. However, we acknowledge that it's both legal and intellectually reasonable to have opinions about laws.
Among the Jews of the 2nd Temple period, the law was not a human institution. They believed that the Law had come directly from God by way of Moses. This Law was central to everything they did, from food and clothing to family arrangements to justice to prayer to ceremony to holidays to anything else. The Law was so central to everything in Jewish life that it took on a massive, overwhelming presence for the Jews of that period. N. T. Wright has described it as a physical presence, something so huge that a typical Jew was barely able to question the Law.
Similarly, the understanding of sin has changed. Today we think of sin as something wrong that we do and then feel guilty about. A sin may trouble a person's consciousness from time to time, but we generally understand that we can choose to pay as much or as little attention to sins as we choose.
For Jews of the 2nd Temple period, acknowledgment of sin was not optional. When they committed a sin, the awareness of that sin stuck around them. They could not make it go away by not thinking about it. It was real to them, like Socrates' Daemon. The only way to remove sins was by the procedures given in the Pentateuch: animals sacrifices, ritual cleaning, days or weeks of isolation, and so forth.
So when Jesus showed up and started telling people that he had altered the law and forgiven sins, he was not merely expressing ordinary opinions, nor even quirky ideas outside the mainstream of Jewish thought. He was, rather, making claims that from the perspective of the Jews at that time were nonsense. He was changing parts of their reality that simply could not be changed. Small surprise that many of the people who heard him thought that either he had gone crazy or that he was working for the devil. In short, that is one of the two logical explanations for the things that Jesus was saying. If he believed that he single-handedly had the authority to rip apart what had been the basis of Jewish society for many centuries, something that no contemporary Jew would even have considered, then either he had lost his marbles or he actually did have that authority.