I believe my concocted scenario meets all those requirements
I don't think it meets the "no reasonable alternative" requirement. In your scenario, it would be perfectly reasonable to remove yourself from the situation and/or inform local authorities so they could get a handle on any violence that may occur from Mr. Megaphone's words before it gets out of hand.
I also don't think it meets the "he did not himself create the danger he sought to avoid" requirement. By punching Mr. Megaphone because you fear his words would incite violence, you have just engaged in the violence you were hoping to stop.
This is probably a whole other huge argument in itself
It is, and I personally don't feel very comfortable with the idea of using potentially lethal force to protect property. However, there are jurisdictions here in the US where it is legally acceptable to use force to protect property and it would be considered self-defense. Whether or not it should be though is, as you already stated, a whole other debate by itself.