Been reading your thread and threads, seems this has been an issue for awhile, sucks and I feel your pain.
Not saying its this issue, but long time ago I pulled my heads, first time ever, to install a cam. Had lots of carbon on the pistons, I cant remember why but I pulled a piston out and found some of the rings were seized up because of the carbon build up, I wasn't using oil, or more than normal. Odd about your plugs, look too clean for a crummy stock coil and basic plugs. keep us posted on the leak down results, that's really the only way left to pinpoint your issue.
• Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a leak at the intake valve.
• Exhaust valve : Air heard hissing out of the tailpipe, turbocharger or exhaust manifold means an exhaust valve leak.
• Piston rings : Whistling or hissing out of the PCV valve, oil filler cap hole or dipstick tube means the air is pushing past the rings. Suspect ring or cylinder wall wear.
• Head gasket : Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.
• Cracked cylinder head : Bubbles in coolant or coolant being pushed up out of the radiator neck can also indicate cracks in the cylinder head or cylinder walls.