Back in the 80's BMW blamed the quality of fuel for a rash of similar problems in their gas engines, but got push back from the gas industry who blamed BMW's "faulty" engineering. So BMW developed their own test, published the results, and announced they would only be covering warranty repairs for owners who used "approved" fuels. Other auto manufacturers backed BMW. The industry was forced to clean up its act a bit by using quality additives. The better brands with higher concentrations of additives above the minimum later developed the Top Tier program to differentiate themselves and to justify their higher cost at the pump.
Sounds like your situation is one where BMW could blame fuel quality, whether accurate or not. In your favor is the fact that, unlike their gas powered engines, I believe BMW still recommends against additives in their diesel engines. IMO, it would take public pressure from statistically significant numbers of diesel engine owners with similar issues to get BMW to cover this type of repair across the board. However, auto manufacturers keep track of dealership warranty stats. That way they know if a dealer is abusing the system and trying to increase revenue at the expense of the manufacturer. If a dealer is a good gatekeeper for the manufacturer (stats indicate below average claims), they have more discretion in marginal situations, to repair a vehicle under warranty. But it's a limited resource so they are more inclined to use that discretion for their better service customers. Poor dealership gatekeepers find themselves being second guessed and scrutinized on every claim. This may, in part, explain why relief for owners with this issue seems so inconsistent.
Whatever happens, were I you I'd certainly want to avoid a recurrence. That would mean sticking to a brand with a good reputation (BP still recommended in the manual?), and buying from local suppliers with newer facilities and good diesel turnover. BTW, just because BMW recommends against additives, doesn't mean there are not quality additives safe for your engine that could help prevent carbon buildup. It just means you're on your own to sort through the dizzying array of choices. If you thought gas additives were confusing, you ain't seen nothing....
