Temporary placement of a fan in front of the internal drive bay resolved the heat problems (which I’ve let run for over a year; stupid me).
When I also had to replace the power supply (it’s suspect in the lockups, not yet proven) I removed the temporary fan (mounted with double-sided tape), and had it come apart in my hands and stop spinning when powered (it was old, it was a test). (Another lockup factor is that it wasn’t getting assigned the right IP address; that made it look like it booted locked, until I logged in from the console.)
And the new modular power supply (200 watts bigger than the old) required me to replace and hence re-route all the power cables. So I messed with the rest of the cables as well (6 flat SATA cables can block some air!).
Temps are back within bounds–but not as good as before, there are two drives up close to 40. Not sure what the difference is from the test state! These darned analog, even physical, things are a lot of trouble.
The order of the gallery is kind of messy. First the photos I uploaded were added in reverse order, and then the light table on which I can drag them to arrange the order is a bit weird, and finally there’s no way short of going into edit mode to see the photo big enough to be sure what it is. So after a while I got annoyed and stopped improving it.