I can feel your pain, Mike...
Anyway, thank you for the teardown of that pen-scope, which strongly reminded me of the
RadioShack ProbeScope I had purchased in the late nineties for USD ~80, if I remember correctly. The ProbeScope was a 20Msps device, with 50ns..1.0ms timebase and 1V/10V/100V AC/DC input range, pen-scope/voltmeter that run on a 12V/20..100mA external power supply (its PCB run on 5.0V). Its accuracy was laughable, but it was very useful for the reverse engineering of PCBs, I used it for.
This is the ProbeScope PCB:
ProbeScope v4.1 (PCB v6.2, 32nd week of 1996)
At the top left of the picture can be seen my botch-work of adding a switchable 9.0 Mohm (= 4.7M + 4.3M) input resistor with a couple of AC compensating capacitors that form a 10:1 resistive/capacitive divider with the 1.0 Mohm device input, in order to expand the 100Vpp max. input range and to become able of using ProbeScope to troubleshoot the CDI/TCI motorcycle ignition controllers I used to repair and upgrade by that time.
As expected, of course, I managed to kill the device by accidentally touching a 300VDC output line of a CDI DC/DC converter while I was probing the low voltage section of the PCB, with the 9.0 Mohm front-end bypassed...
-George