LIQUID EXPANDS AS IT GETS HOT END OF, this is why we have a EXPANSION TANK.
If you take off the cap you lower the boiling point of the water/afreeze mix, and the lack of pressure will allow it to expand easier.
The level needs to be JUST ABOVE MINIMUM, as it designed for expansion not contraction, the colder the water the lower it wioll be but it does not get cold enough here to matter.
Oil in water could simply be that you have inadvertantly allowed some to go down oil drain holes when takinghead off, this will evaporate.
The easy way to tell is to first allow it to get up to temp on driveway so fan comes on then leave it until fan goes off again.
If it does take it for a drive come back do same, same result it is ok.
Any pressure in crankcase could just be blcoked PCV system, have you checked.
tabetha