Personally I am not sure you have established the ECU ("DDE" in BMW diesel speak) is the issue. A dealer tech who says "must be the DDE, I guess" is different than a dealer tech who has looped in PUMA technical support, and with them determined after exhaustive testing it is the DDE.

Remember, that $4k of work has a warranty- *IF** you now go outside BMW for 'ecu coding', you can kiss that warranty goodbye. Even a hit from a dealer tech that 'someone messed with he ECU' will result in BMWNA blackballing ANYTHING that could be blamed on the ecu.

When you have heard 4000, 5000 'stories' about how BMW dealers are fallible, and how they can lead thinks astray, you develop a bit of skepticism about so called 'diagnoses'. Rest assured that if you paid to replace the DDE, and there still was a problem, the next story would be "The XYZ failed also and that was what took out the DDE, so we need to fix that too" (Or that the DDE failing also killed the XYZ)

Just saying.