a. Yes, Mexican coke does use real cane sugar as opposed to high-fructose corn syrup according to the Smithsonian Magazine.
b. In my personal opinion, there is a taste difference between Mexican and American coke. I prefer the taste of Mexican coke and BackyardTaco.com says Mexican coke fans say it has a more "natural" flavor that its American counterpart.
c. Yes, according to the Office of the United States Trade Representative, the Mexican government does impose taxes on imported sweeteners such as high-fructose corn syrup and beet sugar. This is to protect their cane sugar industry but it is considered very unfair and breaks WTO rules.