Hope you get it all figured out and repaired.
Your car must be cursed. Maybe a female at BMW died in the making of your car and she's woman under your hood. She's trying to get revenge.

 

Correct if I'm wrong but these turbo's are oil cooled. How often do you change your oil?? In the N54 motor I would never allow it to go past about 7500 miles. In fact I'd stick to more around 5K. What oil are you using?? Oil breaking down could cause your turbos to fail permature.

Oil's have changed through the years. Mobile 1 0W-40 which is recommended by BMW for these cars(also std. in Porsche and other cars) is an oil that has really been a topic on many forums lately of how poor the quality of the oil is today compared to a few years back.