In my opinion this whole thing about debris magically "hiding" in various cracks and crevices only to suddenly materialize again after a coolant flush is BS. The whole purpose of a flush is to clean out any debris, but that has to be followed up with a pressure test to determine what is and isn't leaking. Other than the dreaded valley pan issue (8 cylinder only), if you've had your water pump, thermostat, hoses, radiator, expansion tank and water-cooled alternator you've done pretty much ALL that can be done for the cooling system. Bottom line, I'd start to suspect deficient (non-bmw) parts and crappy workmanship and I'd bitch like hell until the original BMW shop fixed the problems for good (at their expense).

Good luck!