You should have a clear path for the vapours to leave the engine side pan. The PCV valve will prevent a backfire from feeding back into the engine and blowing the oil pan off.
If you don't have a clear path, then as you mentioned either the fording valve is closed, the PCV valve is backwards, or the PCV valve is gummed closed. The PCV valve should be cleaned once in a while, otherwise any buildup could result in it not closing fully and allowing a backfire to feed into the bottom half of the engine.
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