I decided to cut the mains trace that was connected to the fuse, solder a new wire to the pcb, in series with a "cable" fuse. So no more mains voltage can flow to the battery terminal. The "old" fuse is "out of order".
Then I added the missing connection between pcb "earth" and mains earth/chassis which makes it safe to use now.
It passes the VDE701/702 tests, no ore mains voltage, it's no longer dangerous.