I will provide an honest answer. I trust this will be appreciated. Please remember that a low rating PUNISHES me for being honest and that I receive nothing for my time until a positive rating is clicked.
I would absolutely take the car to a main dealer
the fact that the current shop has to call in a BMW specialist is a huge red flag, they clearly do not know this car or this engine, and you should not use them for anything, period.
you could have a bad cylinder with the car still running okay, but without a proper compression test or leak down test having been carried out, that is impossible to say.
I would strongly recommend you take the car to a main dealer. Pay the inflated price, but you will at least get a proper diagnostic for the cost.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.