I don't know if he would change history but I agree that Clark would have to help, regardless of his initial intentions. Lois would make waves in any time. Clark stops a volcano and doesn't make a headline. Lois stops an altercation and starts the civil rights movement.
Clark could have issues of his own, too. If adopted Kryptonian technology was used in the time machine then a signal from it could have been detected on Krypton, relayed through a probe or something changing the conditions that caused Jor-El to send Clark here. Maybe the whole population would leave the planet or the planet would be destroyed sooner, etc.
I agree that consistency is critical. Asimov was challenged that it was impossible to write a science fiction mystery because the writer could always pull out some gizmo that solved the case. He disagreed, saying that it was possible and that the key is to be fair to the reader by solving the case using the tools you've built into the world. In other words, be consistent. The Caves of Steel is the novel he wrote to prove his point.
