Rarely does the Duramax put coolant in the oil, they typically pump combustion gasses into the cooling system Since yours apparently leaked into the crankcase you are probably are going to need a teardown. Once you get coolant in the oil the bearings go FAST so keep that in mind. I would convert to straight distilled water in the cooling system provided you can keep it from freezing and do a thorough flush of the crankcase until you can locate the issue

We don't typically see injector cups leaking. It could be possible to leak at the water pump shaft I guess if the weep hole is plugged.

As for injector cup leaks it seems they are quite rare and typically man made.