[werk_ag]

On the display, I notice that U1 is not populated. That's consistent with pictures posted with the eBay listing, but I see some other listings do show a device there. Is there any chance that's relevant?

U1 is only installed on models with the touch panel. Its absence will not cause problems.

The peak-to-peak voltage includes the over- and under-shoot, but the traces show the right at least approximate voltage (i.e., 2.5 divisions on ch1, 2 volts/div = 5 volts). I haven't yet dug through my scope's menus to see if there's a different measurement for peak-to-peak that would ignore the overshoot.

I gave a more closer look at the images from your oscilloscope.
If you ignore the over- and under shoot, the signal voltage for Channel 1 is around 4.8 V (seems good), and for Channel 2 is around 3.3V (seems good).