the problems that the OP originally layed out are exactly problems I have experienced with filter drain back. the oil will drain out of the filter to the bottom of the threaded connection. that doesn't mean there "couldn't" be an air leak, but I seriously doubt that pressure would pick back up/normalize by mechanical priming if the system wasn't air tight. I've built milodon swinging pick-up engine combo's without issue, BUT the joint where the pick-up swings can leak air if not always covered in oil and that's why my preference is a static pick-up.