LY5 blown head gasket
#2
White smoke coming from the exhaust and coolant loss means that there is coolant in the combustion chamber. Head gaskets can fail so that the only noticeable symptom is coolant coming out the exhaust so it wouldn't be too crazy if you didn't find any coolant in the oil. Seems to me like a blown head gasket. Other things it could be are a cracked head or block which are less plausible.
#3
Old school where you use a cylinder leak down tester, engine off and cold, remove the rad cap and one by one pressurize a cylinder with air and if you see bubbles in the rad hopefully it is only a bad head gasket. Remove a spark plug do the test, put the plug back in and move to the next one.