As for the damage itself I am still somewhat puzzled as any BMW dealer in the world (even the worst kind) should have absolutely no problem having the damage repaired. All they have to do is have it fixed at their or some other body shop and work with BMW NA Service rep. This happening all the time in hundreds of instances.
Not sure why your dealer has not stepped up to the plate and just get the damaged diffuser fixed (after all it's only a 120$ part & paint).
Just my 2c