just my opinion
Not to sound facetious, but the issue appears to be that the tech. is unable to find the leak. That coolant loss is not due to evaporation, guys.
I recommend you escalate the issue to the Svc Mgr. and/or BMW Regional svc. rep. Also, I would not rule out filing a complaint with the NJ Consumer Affairs Bureau...don't mean to be encouraging legal warfare, but the darn car has a coolant leak!