Cam/Head Swap Disaster.......

So is whatever is showing on the dipstick that same milky color?
At disassembly, is there any possibility that you loosened the intake, head, or TC bolts, and walked away for a while, letting water seep around the gaskets and down into the engine? Any oil change while re-assembling?

If the water is not going down in the rad, then it seems unlikely that you presently have a leak, or it is very slow. Maybe it IS a very slow leak, and the overnight setting did it.

Not sure what to say on the milky oil coming out past the oil filter seal. It takes over 100-120 psi to push those open, unless you just left it a bit loose.

What was done in the process of milling the intake, how much was removed, and why?

IMHO, it has to start coming apart. The timing bolt issue could be possible if one got swapped; it cracks a hole in the #1 cylinder