Thanks for the useful information Gents. I will be checking the other potential contributors of high idle, before revisiting the ISAV's potential contribution:

- as ctaylor mentioned, making sure the throttle linkage is free of binds,
- checking for sneaky vacuum leaks (visually, with a gauge, and strategic carb cleaner spraying) like the inside of rubber elbows and the fittings at the back of the engine.
- checking the throttle valve switch "S29/2" pinouts for continuity
- and applying some RTV to one loose-fitting end of the rigid vacuum line that interconnects the ISAV hoses (labeled 5 in my original post's photo)

After that, I may try a little "bendy-twisty" action on the Idle Speed Control Unit circuit board to see if that gets a reaction and indicates a poor solder connection.