Those are great questions. I’m living in Guangzhou (Cantonese-speaking land) right now, and I’m actually surprised that most of the students from outside Guangdong can’t learn Cantonese very quickly (or at all). It’s probably more a matter of not needing to, but it seems that the tones are the main barrier.

I myself would like to find the following resource (though I admit it would be a very VERY small market): A Cantonese instruction book for foreigners who already speak Mandarin.

I, like you, would love to see an overview of the differences to get an idea of what kind of “discount” I’d get if I actually tried to learn Cantonese.