Could also be coolant (head gasket or...?) but given the bang and instant power loss I agree - turbo connection probably failed and the white smoke is unburned fuel. These things essentially wont move without the turbo.
So how screwed are you? Depends on the shop, parts cost, whether or not they accurately diagnose the first time, etc. This is where labor costs can be the bigger problem. If its just a turbo connection you could probably fix it yourself, even if a temp fix to get to a shop that will work on it. You might not even need parts. Have they even looked at it? Being stuck for months for something like this seems unreasonable.
Have you talked to service folks at Velocity Truck Center
in Sparks? They are a good outfit and give expedited service to people stranded and/or on the road. My experience with Cummins has been like yours - unacceptable.
Velocity is a Freightliner dealer but they work on all makes. They have shops all over the South West and are expanding into the South. My experiences with them have been exceptional and their labor is about 40% less than the typically over-priced Cummins shops.
Service: (775) 331-2550
I really think its worth calling them. Really!