Did you force them to open a NEW work order when you came back in? If you didnt, that let them off the lemon hook....
Really very sorry to hear. If you want some real detailed help, the codes will be essential. Sometimes the code points to a bad part.... but in the case that you have (for example) issues with fuel pressure regulation that trip a misfire in a cylinder, they will see the cyl XX misfire code- and replace that injector or that sparkplug...but the root cause was something else that just happened to trip a code for that cyl. Of course the BMW dealer ought to have a good master tech to really do a more complete troubleshooting. My perception over the years is that 80% of the time these 'trouble cars' are really 'troubled dealers' or bad service and diagnostics...and not wholely BMWNA. A good shop would nail it on the first or second visit, but bad shops misdiagnose, they lie, they obscure, they spin... treat the owner as the enemy. But I guess in the end BMW is culpable- they built the car, they trained the techs, they define how dealers may manage service visits.... the buck does stop...
well, anyway, hope you make some headway. I'd tell them that you want a PUMA case launched if this was not done. (Google it or search here)...if it WAS PUMAd already, you task is to escalate the pressure/incentive on them to pay attention. Dont be irrational, but be demanding.
Keep it as simple as possible...but no simpler.