4. Blockage or routing problem in PCV system
5. Washed down cylinder walls / unseated rings (it's a stretch but possible)

I'm very curious to see what your plugs look like and what the leak down results are. It's possible you got a bunch of oil in the intake from a PCV issue that then got sucked into the cylinders, left deposits as it burned off, creating hotspots which caused some detonation and eventually cracked a ring land. It's pretty thin as theories go but I have heard of it happening. I would think your knock sensors would pull the timing down enough to save the motor though...were they hooked up and working?