My dream is to become a diplomat, what route should I take for my studies?

maybe you should give some thought as to how you intend to use your law degree as a diplomat. that would probably help you focus more on what you should minor in. I doubt a double international affairs/political science degree would add much really. business on the other hand might work very well. be sure to study at least one language as well. it doesn't matter how much you know if you can't talk to the locals.