Well, I had at it a little bit last night.
Pulled back all the wires behind the stereo and started sorting. I came up with a handful of wires that lead to nothing, separated then labelled them. Turned up to be for the Mobile Phone kit and such. Snipped off the openly soldered ends of those, then wrapped it all neatly in tape to segregate them and stop them from being able to touch anything. After that was sorted I was left with the mess that was the wiring job that had been forced upon my car. Most wires had been joined and rejoined at least twice, a few of them were done 5 times. So I went through, snipped them down one at a time, writing down the pairings used, then crimped them all with insulated plugs and put them all back together. Not a perfect job, but a hell of a lot better than what I started with AND there's now no loose or open connections that can potentially touch ground.
I also took a look at the fuses, fuse boxes, and all of the sections of loom I could see in the driver's footwell. Couldn't find anything out of place except the stereo's wiring. Everything to/from the
BEM is in order and none of the boxes show signs of damage that I could see.
Prior to starting all of this I gave the car a try on the whole start/no start thing. Gave it half an hour from d/c the battery to connecting it again, and it turned over. This particular time it took less than 15 minutes to click over and arm itself. At this stage, the trend seems to be that I can leave the car without power to reset the computers and it'll work. So for now I'd just use the car to and from work, disconnecting the battery at the end of my drive. Hoping the car doesn't shut off during my drive
Guess I'll keep you posted if anything changes.