So if a man is hurt by a woman and the world doesn’t stop that harm, is any degree of harm justified and any level of force necessary to end that harm, justified?
Yes.
Society fails at justice
I see the problem. You are conflating "stopping harm" with "justice". There is a massive difference between the two concepts, and we aren't talking about justice here.
Asking police to stop the woman from keying his car is an attempt to stop harm. Asking the prosecutor to charge her with destruction of property is an attempt to seek justice. You described a scenario where the woman is actively harming the man. He is, indeed, justified in using any level of force necessary to end that harm. You did not describe a scenario where the woman has previously caused harm, but is no longer doing so.
Keep in mind that the boy on the bus was actively engaged in harassing his victim at the time his victim used physical force against him. She was not attempting to retaliate for past harms; she was not attempting to seek justice. She was attempting to end the harm he was in the process of perpetrating.