Suppose I can emulate a human brain neuron for neuron. How exactly is it different in capabilities from an actual human brain? What quantifiable property can you assign to the human mind that makes you so certain a machine can never match it?

Your argument is basically good old god of the gaps. You look at what the state of the art in technology cannot do right now, and base assumptions on that without really delving into the issue. 10 years ago you would've included stuff like image classification and natural language generation in the "only humans can do this" bin.