The United States would let Kido Butai turn away

Would the US Navy allow Nagumo to withdraw if he canceled the attack?

Yes. If the Japanese force turned away without attacking, the United States would let it leave.

The response draws a clear boundary between tracking an approaching force and initiating combat against a retreating one. If Nagumo turned away without attacking, the United States would allow the withdrawal rather than force a battle. The counterfactual therefore does not assume that discovery alone makes war inevitable. American action depends on whether Japan continues toward an attack.