Okay: a new full upload on the game V1.1 with the hot fixes is now up – I’ve tested this one by downloading it as a new, unpacked it, then moved my own old save files into the save folder – and everything is working again as normal. As you likely just want to play the game – this is the way to do it.
In truth, I’ve no idea why that happened (yet) but clearly I gone and did a great big dumb with the hot fixes somewhere. Again, truly sorry for the waste of time – the mistakes are 100% on me alone, but I never rest until they’re fixed on the plus side.
Edit: I deleted my own reply instead of editing it for anyone wondering as I'm being very dumb today...