It's harder to involve BMWNA since it was an insurance claim. Many times it's a lot better to get the car fixed somewhere else but with OEM parts. That's what I did, although my case was just cosmetic damage.
Only problem is if insurance doesn't approve a new engine, then it has to be rebuilt, and I'd also want a dealer to do that. By the way, what was done to your engine? If rebuilt, what parts were replaced? Curious about that. And sorry to hear your troubles, but the good news is car is leased. Bad news is you might have more troubles at lease end, no? Hope not man. Please keep us posted.