Where to start? I've been working predominantly with Linux since the mid nineties and with Unix before that since 1977. There is so much to add but the work has barely started. I'm going to put odd items here until enough shape appears out of the mirk that I can try to draw it together.
In the meantime, here are some notes that may help people having similar problems to me.
Utterly unrelated, but it may help somebody one day - a problem cropped up on my laptop with the RedHat logrotate. It's been documented elsewhere that a bug in the logrotate setup can cause logrotate to 'hang' or at least appear to. The problem is to do with wildcards and the logs for mailman. Eventually this fills a directory with thousands of empty files that logrotate keeps trying to rotate. I disabled it by removing the log directory (I don't run mailman) and uncommenting it from the logrotate configuration file. All the same, logrotate would go into what looked a long loop; running it with logrotate -d would cause it to hang when it had processed the last file, wtmp. This gave the clue, because when it exits it updates its status file: /var/lib/logrotate.status. That's editable by hand and when I looked inside it, there were thousands of lines relating to those stupid mailman files. Logrotate was spending all its time reading and updating pointless information about files I don't have. Deleting all those lines from the status file fixed the problem.