First of all, while it's seemingly not important, we might as well set something straight. Those two different models were not built on the same assembly line. They never were. The Fusion is built in Hermosillo, Mexico and the Mazda 6 is built in Flat Rock, Michigan. They share many components, but are largely different vehicles.
Is the rust you're experiencing causing any issues? It may just be surface rust if the vehicle is exposed to road salt or is parked near salt air in a coastal area. You did say severe, so I'm curious as to what the rust has affected so far. The vehicle's corrosion warranty is 5 years, unlimited miles. Does your vehicle fall within the warranty? I realize that it's unlikely, as you'd have a very late 2012 for that to be possible, but the closer you are to it, you may be able to get some type of support from Ford if the issue is serious enough.