...Did you miss? Not all head gasket failures involve a compression leak into coolant or oil.
Rare, but possible - Sometimes when you hear hoofbeats it's a Zebra sneaking up on you.
They concentrate the head bolts clamping forces on the cylinders, because it's the highest stressed area - but if some yutz didn't torque the other bolts out on the periphery (or finds a loose one and retightens it, and shifts the stresses around) that could let the head gasket blow between coolant and oil passages.
I'd still check the catalytic flow - If you pop the gasket between the manifold and the catalytic and all of a sudden it goes like stink, you might have a melted and plugged catalyst matrix - time to tear it down and look. Having a coil pack go bad and dump a bunch of unburned fuel through while running would tend to do it.