When this "boiling" is going on, what does the tempo gauge read?

Once it cools enough to refill the coolant system, does the level drop over night, or only while being driven?

Idea: While the engine is cooling down, remove the coils and ground them,(so that when you crank the engine you "might" not get a missfire fault). Leave the plugs screwed in the head. Top off the coolant and leave the cap OFF. Have an assistant crank the engine over and observe the coolant expansion tank. Does the level rise and fall slightly in sync with the engine cranking, or does cranking cause bubbles to come out? Either would indicate a blown head gasket. If you could pressurize the coolant system somehow, remove the plugs and see if it fills one of the cylinders with water.

"...Is there any way the turbo could be the source of the loss?..."

That's a good thought. I guess your car has a water-cooled turbo?? If you are leaking internally into turbo, I don't know any easy tricks to determining that, other than disconnecting the exhaust and intake plumbing to see. I'm not a turbo expert, but it seems to me that if there is an internal water leak that it could be passing into EITHER the exhaust side or the intake side. Maybe someone else will read this and comment.

"...Is there any way a bad thermostat could be contributing?..."

Probably not if the temp gauge is staying in reasonable range. But how about the coolant cap? Is it known to be good?

Hang in there...