Sounds like more than a cup. Can you hear the compression gasses huffing into the crankcase if you remove the oil fill cap and have someone crank it?

Compression test is still manual, laptop wont say anything, have to remove injectors. 300-400psi is normal for a Duramax but most important is that there should not be more than a 50psi variaton between cylinders.

The LB7 pistons were strong but theres still a chance you cracked one or put a hole in one. With the black smoke before it died and the fact it wont start I actually wonder if you have an injector stuck open.  If you crank it now does it puff black smoke?  Some injectors will torch pistons, some don't but having one stuck open will drain the rails and not allow the truck to fire. Not doubting your shops mechanic but you would be surprised how little experience many of them get even if they sell a lot of them.  I get a lot of my parts from our local dealers and I am always amazed how little work they ever end up doing on the diesels because they usually get traded then go to auction.