Epilogue: the #Amiga 3000 is back in one piece, fully restored including the battclock (Amiga parlance for RTC).
After #retrobrighting and some cleaning, the case looks new. Actually, one of the 4 external screws is now missing, d'oh!
After some testing, the Workbench partition became corrupt: the Tools directory was still shown in dir output, but you couldn't read its contents. A DiskSalv 4 didn't find any issues.