Not sure how I am confusing everyone so badly with this one.

I poked the hole in the clogged, oil drainback tube, deep in the block.

It was clogged completely.

I jammed a steel rod down there to see if anything would happen and eventually it broke through into the oil pan.

Having that blocked should NOT have pressurized the block as the entire thing can vent right out the separator box, out it's big tube and right into the intake snorkle.