Two things.

One Ive never seen oil/water mixing both ways in a motor with a bad head gasket or such. Either its in the coolant or its in the oil. Seems like it would flow back in forth but I have never seen that happen.

Two the milky stuff on the cap is common if you dont drive the car long enough. My bmw's get it allot since Ive got so many of them they dont get driven much.

I would get a radiator/coolant pressure tester and see if it hold pressure. If it does and the oil looks fine when you change it I wouldnt worry about it if it was me.

If your not loosing coolant you dont have a leak.

The final thing to try if you are positive you have a weeping condition and not an all out leak you can use bars leak silver. Its used in cadillacs and and gm vortec engines from the 90's from the factory. I dont know if they still use it in the current stuff but it wouldnt surprise me.

The castings on the heads of the 90's gm stuff was so bad that they dropped a block on the bars leak stuff in the radiator on the assembly line. That part is trivia. But the problem that people ran into later as the vehicles aged and had coolant changes the bars leak went away. Then they started having problems weeping water into the cylinders. Fresh application of bars leak fixes the problem.

This only applies to the bars leak silver.