Alternate title: PROGRESS. I AM MAKING IT.
I still have no idea what the original problem was, but I did manage to work around.
And I managed to work around because painting the normal map in the paint room was hell and I also really, really wanted to solve the other problem.
Because I'm not the sharpest tool in the shed, even though I realised the 3d Coat menus were contextual, I didn't really realise they were contextual. So for a long time it didn't occur to me that I could export the sculpt mesh. I got there eventually though, and sent it over to Blender to see if I could figure out what was wrong with it.
Couldn't find anything wrong with it (not that I'm a pro by any stretch of the imagination) and found out I could export the model without reducing it first and reimport it into a new 3d Coat file without things crashing and burning, so I did that, imported the retopo model and then baked that.