With a blown out head gasket, I would expect there to be coolant in the oil and steam coming from the exhaust pipes?
I would also expect it not to run too well.
The bubbles in the coolant overflow could just be from air being caught in the cooling system? Maybe the hose between the radiator and the overflow tank got pinched-off and that's why you couldn't see coolant in the radiator while the overflow level was OK?
I really have to wonder if the water pump or thermostat isn't the problem???
Since the fan is working, it should be cooling the radiator.....but if it isn't circulating the coolant (thermostat or water pump problem) then it's going to overheat.
Since it runs OK, I'm thinking more like one of those two things being the problem. It's easy to check the thermostat by removing it. If the thermostat is open at room temperature, it's gone bad. Then also putting it in a pan of water on the stove with a thermometer in there. It should open-up at 88 degrees C.