Coolant appears to be mixed with oil

I would stay away from any repair fluid because it really does not know where the leak is located, it just clogs everything including the tubes in the heater core and that brand new radiator and may not even clog up the head gasket leak.
I would though go ahead and use the combustion gas leak detector.
Are you saying that when you remove the engine oil fill cap, there is a milky like substance on the underside of the cap?

If you see only the crud on the cap it is more likely from moisture in the air getting into the engine and mixing with the crankcase vapor. I would start with draining the oil, any coolant will come out first as it will settle to the bottom. If you do not find any the next step would be to do a pressure test on the cooling system. If it holds pressure it is not likely you have a leak and the goop is condensation. (Pressure test kit is a loaner tool at many parts stores)
It is common to see that if the vehicle sets unused for a while or if it is not used for longer trips that get the oil hot.

If you are really worried you could do a combustion gas test, those are also a loaner tool (you pay for the liquid though).

Can you post a photo of the underside of that oil fill cap?

Setting for that long will build up condensation in the engine. When you start it up the water heats up along with any of the fuel vapor and other light contaminates in the oil. That circulates through the PCV system and will collect on spots that disrupt airflow.