Coolant leak - HG replacement?

[wishful-fish519]

Hi guys,

Please excuse me, I know this is a commonly discussed topic but I'm struggling to find a plausible answer that doesn't lead to more questions.

My Seat Leon 2014 1.6 TDI is leaking coolant. Here are the facts:

1. Coolant leaked periodically and had to be topped up monthly.
2. Expansion tank cap flew off while driving ~90km/h and was replaced for a new cap - issue persists however the coolant doesn't leak periodically now but upon checking the reservoir prior to driving once; the entire coolant tank dropped all it's fluid and there was ~2cm of coolant left, way below minimum.
3. 184k km on the odometer.
4. Timing belt & pump changed at 173k km (-13k km).
5. EGR checked by mechanic, dry.
6. Intercooler checked by mechanic, dry.
7. Vehicle checked by mechanic for external leaks, dry.
8. Coolant system pressure-checked by mechanic and no pressure drop.
9. Driver vent doesn't blow hot for the 1st ~15min, proceeds to blow hot for 30min, then goes back to cold for the rest of the journey. Passenger and rear vents blow hot (after the coolant/temp gauge reaches 90 celsius.
   However...
   1. No sod on the windshield.
   2. Car doesn't fog up by itself mid-journey.
   3. No smell of coolant in the vehicle.
   4. No wet carpets.
10. No white fumes from the exhaust after running.
11. Car does not overheat, coolant maintains a stable 90 celsius and engine oil 90-108 celsius.
12. Engine oil checked ten-fold and revealed no signs of coolant leaking in.

Sum-sum my mechanic declared that based on the above; it is not the EGR, intercooler, thermostat, coolant reservoir, coolant hoses, or heater matrix.

I seen @SuperV8 post here that a DPF regen with a blocked heater matrix could cause coolant loss. I do recall driving 800-1000 km on a motorway with no leakage, followed by 2 days of city driving - which resulted in the 2nd time the entire coolant tank dropped all it's fluid in point 2 above. However, my mechanic stated it is not a heater matrix issue due to point 9 above.

I am out of ideas and my mechanic declared this an internal leak and therefore recommended we replace the head gasket. I spoke to multiple people that have mentioned this might be unlikely due to points 10 through 12 above.

Is there any alternatives anyone can recommend? Should I consult w/ a 2nd mechanic? Is it safe to drive on for the time being (I believe this issue has persisted since I purchased the car ~8k km ago)?