AeroSport received my cylinders and emailed me their findings. The #4 cylinder (same one with the broken ring) has an exhaust valve and guide that are worn beyond limits. This is the cylinder that was forcing oil out of the breather and turning the oil dark so quickly. After the last oil change it was black in two hours.
They're going to rework the other valves and seats and hone the walls. And after talking to them on the phone I've decided to replace the pistons and pins and new rings all around. My pistons were within limits but did show and measure some wear. And while they have the cylinders I asked for a port and polish.
AeroSport has been excellent to deal with and they're telling me the cylinders, with pistons loaded, will be on their way back to me by the end of the week!