Some interesting stuff there, would be good to pick your brains on a few points.

The no gloss over metallic. Are you referring to Commodore here? You are correct in saying the only factory two-tone metallic paint (that wasn't joined at a dividing strip) since HT Holden was the VC Shadowtone SL/E's. We've being trying to figure these out for a while now to make sense of how they were done so that the two tone HT metallic made some sense. The use of a non clear over base for at least one of the colours helps to understand how these may have been painted. Back to HT in the two-tone metallics for a second (HT GTS in Monaco Maroon or Verdoro Green with gold metallic stripes or HT Brougham with the metallic silver blow the stainless trim), the cars were dipped in black goo, then they had red-lead electrostatic primer sprayed on, then rubbed, then the colour. For the other colours the satin black was applied in the finishing area, but for these the metallic gold had to go on prior to the clear so the metallic was protected. Survivor cars today show that when hand buffed no colour comes off either the body or the stripes unlike the solid colours. The logistic problem with these is the acrylic used in HT was heat cured, so the car was hot when the paint went on and it was baked afterwards. So when and how did the masking happen for the gold stripes? This we cannot explain, and it also probably explains why it wasn't attemped again until VC. So the availability of a non-clear over base metallic may explain the VC's, we do know the top colour was done first.

The slops black you refer to was only used in some places, and as you say had no refinish code (but it did have a GMH specification - S113). On HQ Sandman for example it was used on the following:

Underside of front fenders and wheelhouse assembly.
Front end frame complete.
Radiator Support panel assembly.
Also used on the chassis an misc parts on the cab-chassis.

There were a few other blacks - 599-00364, used in the plenum, dash panel and front body pillar outer.

And there was 578-04149 used for all body blackouts and stripes that were painted. This is what GTS sedan door frames were painted in.

Pagewood had a small parts painting area (reportedly with a water wall) for painting all the small bits as you describe. This is probably where non-black wheels were painted, and other bits that needed a "better than cr@p" finish was required. Would be cool to know if you saw this in your travels!

The overspray all over engines was how it was, they are all like it. Rough as guts! They wouldn't have done much touching up in engine bays from HJ onwards as these were all sprayed with a clear lacquer to make them "prettier".