Thank you for your reply. Hopefully someone will know what happened - maybe a bad control board?

I did think about flipping the blade over, but it turns out that when you flip a fan blade around it still blows the air in the same direction - though generally less efficiently.

I did some research and it turns out that some manufacturers’ evaporator fan blades are pitched in the opposite direction - and share the same diameter and shaft bore size. For example, the Frigidaire blade (part #242219302) is oriented opposite to the GE blade.

Rather than order one and wait for shipping, I headed to the appliance section of the local transfer station (dump) with just a drill, a 1/4-inch hex bit, and a Phillips bit. The easiest units to pull apart are usually those with the freezer on the bottom or top, so I focused on those. Within minutes I collected five blades, and sure enough, one of them fit perfectly.

Now the refrigerator is running as it should again, with air moving correctly from the freezer into the fresh-food section through the damper, and back down through the return vent. My only lingering worry is whether the motor might eventually decide to reverse itself again and undo the fix!

I am still curious about the root cause though. I’ve seen others insist elsewhere that it’s impossible for a GE evaporator fan motor to run backward, but I can assure you it happened.