"Modelled" would make for less of a strawman than "copied," but even "modelled" isn't at the heart of the Christ myth theory. Models are simplifications, but the Christ myth theory is that Christianity was synthesized from several sources, including Jewish and pagan ones. So the headline question should be, "Was Christianity Synthesized from Pagan Myths?" That word "synthesized" allows for multiple sources. Even more precise or helpful would be, "Was Christianity a Jewish Version of a Greco-Roman Mystery/Salvationist Cult?"

I think many Christians would answer your second paragraph by saying that Jesus's death and resurrection (the Pauline part) superceded his teachings. His teachings and righteous acts were meant to prove his divinity, not to serve as a legal code for his followers. Once Jesus died and resurrected, a new path opened up for our salvation, and the Christian's obligations were less onerous (than the Jewish ones). They need merely believe in the power of Jesus's resurrection, as Paul said.

So when Paul said that Jesus's death and resurrection made "the Law" obsolete, he was implying that they also made Jesus's earthly life so, including his teachings and actions. That's why Paul doesn't talk about the historical Jesus, and this is why the epistles come after the gospels in the NT even though the epistles are older.