lol, your machinist is an idiot.
Its caused by detonation. The first place detonation goes for depending on the sevarity of it is the top edge of the block. Which is why your headgasket pushed out and broke. Im sure the cylinder head in the same area by the firing ring has the same damage maybe just not as bad. Lol. Coolant hahahaha.
Next place detonation attacks is the ring lands. Your motors probably had just slight detonation if you didnt really notice any changes to how the motor ran other than you started losing coolant or pushing coolant out of the radiator under boost. Heavier detonation will do that and break ring lands. Im sure your pistons around the edge as well are pitted and eaten away. Detonation, detonation, detonation. Been there done that.
Most likely caused by too much boost, too much heat being left in the cylinder creating hotspots and too much timing can be related as well as a/f ratio. Many things can attribute to the detonation.