Fact engine is still running is a good sign. Maybe just a blown head gasket. Or it just barely running with low compression because a piston or two have melted, along with the head sending molten alloy into your turbo and that why the gasket is doing nothing anymore
Answer is get the head off and look see.
RAVE can be found via google- not right to post details on this forum.
Water wont go to the rad until the thermostat opens when it's hot enough .
Depending on where you travel you can drive around for weeks and the tdi engine not need the radiator but one day you drive a very long big hill and that's when the radiator gets used - if rad is internally blocked or the thermostat is faulty then engine will melt rapidly - temperature gauge shoots up into the red for a moment and drops back down to normal as the temp sender is no longer sitting coolant. anyhow.... just saying the Radiator not getting used symptom you describe might be normal at idle.
Oil in the turbo inlet - there is often some oil present there depending on the mileage of engine. How much oil in the inlet is the thing. If your missing oil is truly going that route then do not use the truck as there is a risk of engine running away fuelled by engine oil.
If the header tank is pressuring, bubbling throwing out coolant, very hard coolant hoses etc then exhaust gases are getting into the coolant system via the head. Possible bypassing the head gasket because it is has blown. Maybe you have some oil in the coolant. Oil leaking via a fractured gasket into the piston chamber too. BTW does the engine have rusty coolant stains behind the alternator streaking down side of the engine block?
What's the exhaust smoke like?