Why don't you start looking/quantifying coolant loss?

Since the system is "closed", the coolant loss will be very small; maybe a cup in a few months if that. If you are getting coolant going into the oil, then you should see evidence of the loss at the overflow/fill reservoir.

Also, coolant that gets into the oil will emulsify into the oil partly because of the heavy churning action in the crankcase, and partly because the glycol component in the coolant is less hydrophobic than water.

It doesn't take much water in the oil to change it to milk chocolate syrup appearance. A few percent coolant contamination is enough. That equates to a quarter to a half cup in the oil.

When you have emulsified oil, you can readily see it on the dipstick. It is not transparent, but murky & milky.

The chemical test for exhaust gases in the coolant is a pretty sensitive test, but it's testing a different thing. Even if the chemical test shows some weak exhaust gas presence in the coolant system, a good mechanic will usually wait for a more definitive sign before saying it's time to take the head off.