The local dealer has been very diplomatic, choosing their words and questions carefully. They've never said anything accusatory, but I suspect their first impression was that we had done something to mess things up.
My understanding is that the fuel was one of the first things tested. It checked out fine.
Another question they posed: Had the vehicle been driven through high water or submerged? (No.) They said it didn't appear any water had made its way into the engine. Sounds like they just wanted to hear a corroborating answer.
We were also asked if any work had been done, presumably implying some sort of software or firmware modification. Subsequent electronic diagnostics confirmed that nothing unusual had been done to any engine programs.
I don't know how rigorous the local tests were compared to what can be done elsewhere, but BMW seems to be satisfied that the failure wasn't in the customer's control.
Truth be told, I have opened the hood only once since taking possession. I was underwhelmed. Nothing fun to look at, really. My involvement has been pretty much limited to driving, washing, applying Zaino products and refueling.
Speaking of refueling, the fuel efficiency is better than I had expected (and improves with age). Before the Event occurred last Sunday, the vehicle was getting 28.6 mpg on average for the entire road trip. At one point during a long descent off a mountain pass, she enthusiastically reported a total estimated range of 847 miles (141 driven + 706 range to empty). A bit optimistic, but I like her attitude.