Sounds like a good one... ugh
Ok, head gasket, possible, but rare on the 7.3 / 444e, Cracked head, seen a couple, but once again, pretty rare, these heads are built fairly well.
Cracked cylinder wall. Very high chance of that one. From my experience, # 7 or #8. Most I have seen have been #7.
Now, this is not to say that a simple solution won't be found, just voicing some experience here. The basic design of the engine allows combustion gases into the cooling passages at the cylinder walls, head gasket surface, or.... The injector cups if something is horribly wrong with one of your heads. It would take a combination failure of the seating surface in the head, a bad copper washer, a loose injector clamp, perhaps a blown off injector tip, and perfect alignment of the planets... but it COULD happen.
Best thing to do, is pressurize the cooling system with it full of water, and see where the water comes out. Drain the oil, and watch to see if water comes out of the crankcase. While you're at it, pull your turbo, and see which up pipe is clean, or has white residue, that will tell you which side of the engine to look at.
On the t-stat change. These things normally self bleed quite well, but some of them can be an absolute bear to get the air out of. And when they do burp, they spew coolant everywhere. Best tool to use is a vacuum bleeder on the overflow tank, it will let you get all the trapped air out, then draw coolant in to fill the void.
Good luck with it.