This man's an optimist!
The washdown theory seems plausible.. I don't think antifreeze separates out from water. Was there any milkiness in the oil? That would indicates mixing while operating, as opposed to the water coming in separately.
I'd 100% surely pull the valve covers at least, and look there. And then squirt a teaspoon of oil in each spark plug hole and rotate the engine by hand with no plugs in. If it is stuck, or rotates super hard anywhere, then you darned well ought to investigate more.