Can't help you with interpretation of the signal traces. However, you can try a very simple test. While engine is running, unplyg the MAF sensor. Do you get any change? If the engine runninv/stumbling just as before if means your sensor is bad.

 

UPDATE 28

In fact, the first test (just before what I posted above) was probably messed up by a loose wire. The behavior was similar but worse. The engine sputtered and stalled in a few seconds. The check engine light came on and the computer reported a MAF error. That's what prompted me to make new, better fitting wires.

Speculation:

Looks like the reason the computer doesn't report an error (when properly plugged in) is that the voltage is within range of normal behavior. Not sure how the computer figures out the (possible) square wave without looking at the algorithm, but it makes sense that higher frequency/more pulses per unit time would be interpreted as more air/higher RPMs/more fuel required. The flat line at 4V that I see is just the extreme of infinite pulses, telling the computer to dump as much fuel as it can, thus the horrendous mileage, and misfires due to too much gas, but that doesn't fit the flat line of the O2 sensor, which seems to imply too lean, or is it the opposite?

Result:

Based on the above, I ordered a new MAF sensor. Just received it and installed it. A preliminary test seems to indicate that this was the problem. Upon cold start the engine now goes to 1800 RPMs (it used to go to 2K) and as it warms up it goes down to 800, then 600 and occasionally 400, but it doesn't stall any more. The misfires are now very occasional and occur only when flooring it at idle. I'll have to drive it around and let the computer adjust to the new data based on real driving conditions. If I'm right, the computer will eventually optimize fuel and RPMs. I'll report back on mileage once I have emptied the tank.

Thanks to everyone that offered suggestions.

 

UPDATE 29

The computer has adjusted. I now see the old behavior: 2000 RPMs when starting cold, no more misfires, and the mileage seems back to normal. Idle when warm never goes below 750.

The misfires that remained were obviously from old/bad gas. Now that I've refilled with new gas, the misfires are gone. Mileage is back to normal. Haven't used up a complete tank yet, but it's on track to get the 350 miles I was used to.