My coffee machine is working again. It seems the water that I found in various parts of the machine was probably the problem. Based on my diagnosis (which could well be flawed) the waste water bottle being full caused the issue. But if the problem occurs again I’ll have to go back to the drawing board on that one.
I did get to pull the machine apart again and used that as an opportunity to comprehensively photograph its gizzards.
My desktop PC and Breadmaker are both still out of commission. Hopefully not permanently.