Ah... all works.

A poor functioning relay, not the stalk switch, was the issue.

The original BLACK dipper switch is unused. The RED does in fact go to the relay, and the IPD instructions were not clear enough on the which grey wire goes where, but I eventually got it sorted out.

I switched on a similar 8 prong relay from my parts stash (i.e. precise origin unknown - maybe the old 1800E) and suddenly it all worked just fine. I added a return wire from the dipper red back to a restored indicator light and my high-beam indicator function returned.

Just in time too. I just got dumped on by a major thunderstorm.

Now I just need to sort out my turn signal weirdness (no dash light, slow function despite having a new relay, but the hazards still work great) and the now dead oil light indicator...