When you get your old rims back, I would revisit this issue with your dealer and if no results, insist on a discussion with their corporate rep. If there is a metal defect with the rims, they can certainly be tested and I would demand it.
On a side note, I recently hit a water rut 2ft across and a 1ft deep at 50 mph with enough force to dislodge my clip on tank bag with no damage.
To get any results with BMW you should exhaust all steps within the system. Like them or not, the key player is going to be your dealer. Talks of lawsuits or going over some ones head is just going to force them to marshal their forces against you. Statements from others, pictures of the crime scene and even referral to some of these threads of other's misadventures will be helpful. I know you are upset, but your best results will be achieved by being calm and business like.
If all that fails there will be plenty of time to blast them on the internet. At least that is what I hear