Since SA won't help, say you'll be elevating to his boss to see what he thinks.
1) call GM of the dealership / service area, come in personally to talk to him and explain what's happening (calmly but firmly - you'll get taken more seriously that way).
2) request for video that to see if you can prove no damage when dropped off.
3) if all else fails, see if BMW NA corp. can do anything for you. Long shot though.
At minimum, you've shared some of your burden with the SA who should get some negative feedback on it from his boss.
Net, the problem is you really did take the car from the SA without looking and they can easily point to you having damaged your own car (unless service bay video says otherwise).
Sorry you have to go through this, but you live and you learn as they say...
AWE NR | SWIFT SR| PAGID RS29 | CANFlap | 6MT