Many thanks to Jim and Brian for the thoughts.
The mac was restarted several times but did not make any difference.
Brian's comments were helpful as I had not realized that the first warning message was strange. There was no company logo on it - it was in plain text in a plain box so I do not think it came from either Mac or MC. It suggested that the crash had somehow invoked another program to start up - unlicensed as a trial perhaps? I did not pay too much attention as I thought a restart would clear it away. On reflection, I think I am right in saying that the trial MC gives you full access so it does not appear to have come from MC.
I am running MC20 from the internal hard drive (OK a slow spinner) of the iMac. The library is on the HD. The output feeds a USB dragonfly DAC. The music files are on a network connected NAS via cabled ethernet. Nothing too demanding here. I am not running from File Vault. Just one admin account running.
Your comment on a virtual machine made me think about where the ghost program might have come from. I have Parallels installed (latest version 10.2.2) to run XP and Win7 from time to time - but no MC on these virtual machines. All was running well up until yesterday when MC crashed. Parallels was not running but had run the week before.
I started Parallels up and within a few seconds, the mac had a kernel panic. I don't see those very often. From there on in, the Mac would not start up past the apple logo. Clearly the initial crash had led to something major and Parallels was involved even though not actually running. At this point I chose the sledgehammer route and opted for a complete rebuild from Time Machine.
Several hours later, everything is back to where it was before the crash and all is well. This has naturally removed all the evidence of what happened.
The trigger was the force quit on MC 20.0.126 and this seems to have led to series of worsening events in the OS, possibly made worse by the presence of Parallels. The force quit arose from the spinning beach ball after getting an album to 'update tags from the library', that happens from time to time. Normally all is well after an MC restart.
I don't think we will ever know what happened and I can only hope it was a one-off deriving from my setup. Before I discovered the corruption, the MC behaviour of not being able to hold registration was very strange and I leave it on file here in case it crops up again elsewhere. I did all the mjr loading processes but to no avail. MC would accept the registration and run but would not hold it. Unfortunately, I was not able to move the suggested directory before things went truly haywire.
My MC master mjr file was still valid but installing via 'Package Installer' seemed to invoke a restore request, each time. This may be another indicator of the corruption process.
Thanks for the ideas and support. Your advice gave me a way forward. And as ever, good backups on everything prevented a problem becoming a disaster.
Mike B