Since my last post, I have spent so much time and energy investigating this... and now I can finally provide an update and a conclusion to this saga!
At one point, I decided to pay for 3 days of access to Service Info on
acdelcotds.com to ensure I had the most accurate wiring diagrams available.
Making the most of it, I put in my VIN and started browsing through Service Bulletins when I came across #18-NA-270 (Document ID: 5138979). This perfectly matched my problems, from the error codes and modules they’re observed in, through to only happening when hot and under brakes.
At this point (first time I’d used the site) I noticed I had my region set to North America - so my VIN coming up as a “Camaro” wasn’t “a quirk” as I initially thought.
After changing the region to Australia/NZ, my VIN correctly identified as a Commodore. There was a similar Service Bulletin (#17-H-030; Document ID: 4855165) listed, with the photos in it now matching what I’d expect.
The bulletin describes a “rub condition” on the front of the engine which has obviously been known to wear through the wiring harness there.
Using mirrors, and a crappy old borescope camera, I tried my best to inspect for any damage, but could not see any. So, I just pulled the wires as far away from the engine as I could. (One of the steps in the bulletin.)
I went out for a drive, and I couldn’t believe it. For the first time in as long as I can remember, no dials dropping out, no glitching, no “Service ESP”, no other random messages, no issues whatsoever! The “Check Engine Light” also switched itself off.
To be sure, I wanted to give it some time.
It has been over a month now, and the issues have not happened since. So whilst I still need to find and fix where it has rubbed through, at least I know exactly where the problem lies!
It really surprised me, because it’s obviously happened enough to make a service bulletin about it, but nothing of the sort turned up in any searches I did (nor did it occur to the dealer who had my car for three days)
So, hopefully this here will add another possibility for anyone experiencing this dreaded issue.