Update on the current laptop situation: @deshipu opened it up, looked it over with the binoculars and applied the sysadmin touches and … and he found a lose screw wedged into a corner of the laptop's innards so that it wouldn't rattle and betray itself. We recovered the screw, located an empty place that looked like it might be missing a screw, put it all back together, and it worked!
And then we switched the disks of PowerBook Pro and Purism laptops. I'm writing this message from the laptop I feared was toast!