The air sound coming from the oil filler while doing leakdown indicates cylinder/piston/ring problems. It could indeed be a stuck ring, but I see this as unlikely. It is, however, worth pursuing.

Is there any coolant in the oil, or oil in the coolant? I would be surprised if there is any head or head gasket problem from overheating. These heads are big, solid and cast iron. If there is no coolant in oil or vice versa, then you might try the following.

Using some sort of magic medicine for freeing stuck rings should be approached with caution. A diesel engine has very small combustion chambers, so there is not room for much uncompressible liquid. I would pull the injectors and squirt plenty of Marvel Mystery Oil in the number one cylinder and let it sit overnight. The next morning, turn over the engine until it is no longer blowing oil out of the injector hole. Because of the prechamber, it will be hard to get the oil in and out, so MAKE SURE all oil is out. Then put in your injectors, bleed and start.

I think this is worth a try.

Good luck,