OK, I put it all back together.
Good news is that the speedo now shows speed.
Bad news is that the trip meters still reset every time I switch the bike off and the odometer display is still kinda dim. Perhaps the constant power to the speedo (purple wire) from the battery is the issue there. I cleared the P1500 error code and started the bike several times without it returning but then eventually it came back. So there's still something wonky going on in there.
While I was at it, I also measured the frequency of the pulses from the road speed sensor.
ECU to speedo: 0 - 3V square wave pulse every 90ms at 20kmh (11.1111111... Hz).
Road speed sensor to ECU: 0 - 5V square wave pulse every 2.2ms at 20kmh (454.5454545454... Hz).
Didn't have a frequency counter, just eyeballing it on the scope so it's somewhat approximate.
The pulses are symmetrical for both. That is, they spend the same time high as they do low.
The next test will be to see if the speedo is accurate. Given the revs and gear I used on the stand, I've no reason to doubt it. But we'll see.
I guess it's some progress anyway. Going for a 4 day ride over the ANZAC day weekend (23rd - 26th April) and will hopefully get an SS1600K ride done then.
At least it's rideable, just gotta rewire the GPS into it beforehand in case the speedo fails again.