wow thats crazy that they denied warranty. did you try another BMW dealership ? i've come across situations where one dealer would deny warranty and another would approve it for repair. btw, doesnt bmw have a rust perforation warranty ? i am not sure if it covers the subframe of the car though.
and thats a lot of rust for a car with such mileage. i live in montreal and i have 50k kms and my car doesnt have nearly that much rust in that area. granted i tend not to drive it as often during winter and always give the underbody a good rinse with fresh water every few days if i do drive during winter.
Current: 2013 F10 M5 & 2013 GLK350
997.1 GT3RS (gone)
2009 WRX STI (gone)
e92 M3 (gone)