Dunno if this helps, but have a look at part of the frame that is not exposed to the elements e.g. under the seating or up around the fuel tank. You may find evidence of corrosion patches in these areas, or blistering/scab formation under the paint of the frame. If so, it may be due to either a contaminant on the frame surface before it was painted, or a failure of the paint process to adhere properly to the frame, an you can show this to BMW.
You might also want to try and remove some of the paint in one of the above areas with your finger nail - this could imply undercured paint during manufacture.
Looking at your photos, there is also a large amount of corrosion on the wheels. These would be manufactured completly seperately from the frame, and to a different process. Corrosion on the frame as well as wheels may be down to you having unwittingly driven through some corrosive spillage on the road. I saw something similar with a relatively new car, and the guy got an opinion from the manufacturer, which helped him get a respray via an insurance claim.