The clogged cat theory sounds good. Is there some special OBDII test that could verify this via O2 readings or increased fuel delivery? We'll do a smell test on the oil and see if we can detect any gas. Dad suggested having a used oil analysis done to confirm blow-by, but given how much fresh oil we're adding, would the oil stick around long enough to be worth testing? Given that the cats run about a grand just in parts we'd like to be sure they're the problem before buying new ones.
I've heard of the Maxima guys punching out their cats and putting the second O2 sensor on a spark plug anti-foul spacer to make the computer think everything's normal. We have no emissions testing around here obviously, and given how much oil's going through this thing I'd be surprised if the cats aren't in a bad way already. Even if the computer threw a low catalyst efficiency code, would that make it quit dumping gas?
I would like to check the valve screws just to be sure. Two questions on that. First, if we pull off the upper intake and find bits missing, should we assume the motor's shot? And after checking/loctite, can we get away with re-using the gasket? If we do solve the oil consumption it'll likely get new intake gaskets and a new PCV just to be on the safe side, but until we know there's hope for this engine we'd rather not throw new parts at it.