Any diesel savvy members?

[1971'Burb]

So, my nephew has this 2015 Duramax 6.6 crew cab. It is giving him some issues with the DEF system. Speed is about to be limited in another 55 miles or so. Dealership claims it's an actuator in the exhaust system of some sort. No further details were given. I suggested he deletes that crap and buy a tune to match but that would void his warranty. It cannot be hidden. The dealership will remove ECM and send it off to Chevrolet for dissection. They find traces of the tune and warranty is no more.

Question:

Is it possible to aquire another ECM, program it for the vehicle it's going in, tune it and swap for the factory ECM while it's stored safely?

Seriously

The dealer repairs costs 100'$ and the tune will fix these issues.