Thanks, Geoff!
Yes; it seems to have worked. The display and audio are all as I would expect 
I know what I have to do with my entire collection now; thanks! There is a ‘but’ though. May I ask for help with that, please: (apparently?) ‘stuck’ scanning 
.
Does that mean that Roon also took care of the albums which I had previously (and erroneously) merged?
Definitely (thanks for the instructions). I also restarted my machine.
Unfortunately, I have run into this phenomenon again:
So I tried removing albums (now correctly-named) which I thought might be causing it (one in particular with eight tracks/files because that was the delta… it hung at, say, 24/32 and 56/64) when I did a test rename as you suggest and I hadn’t stopped the server, but Roon wasn’t running; then Rescanning etc and Library maintenance > Clean up Library.
But it’s been at 315/590 (Fast 4 cores) for over 30 minutes.
I think I could probably make that go away by switching to ‘Off’. And restarting etc. But I don’t really want to do that.
Thanks in advance for any suggestions, please.
Logs again?