Last piece of information for tonight: re Inkscape, my correspondent over in the Inlscape forum gave me these two commands to gain more information:
ls -al ~ | grep config
ls -al ~/.config
This is what transpired:
Last login: Sun Feb 15 00:32:45 on console
Michaels-Mac-mini:~ MichaelGraubart$ ls -al ~ | grep config
drwxr-xr-x 3 root staff 102 28 Oct 22:01 .config
Michaels-Mac-mini:~ MichaelGraubart$ SELECT ALL
-bash: SELECT: command not found
Michaels-Mac-mini:~ MichaelGraubart$ ls -al ~/.config
total 0
drwxr-xr-x 3 root staff 102 28 Oct 22:01 .
drwxrwxrwx+ 54 MichaelGraubart staff 1836 15 Feb 00:54 ..
drwxr-xr-x 3 root staff 102 28 Oct 22:01 SMART Technologies
Michaels-Mac-mini:~ MichaelGraubart$
It turns out that 'SELECT ALL' is some administrative matter of the Inkscape forum and not part of any of the Terminal commands, so please ignore that. But I did indeed (presumably on 28 October) install some SMART software: SMART Notebook and associated bits and pieces. Could that somehow have usurped the creation of a .config file? Should I look for it by searching invisible files? What should I do?
And above all, 'Warning: SUID file “System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/MacOS/ARDAg ent” has been modified and will not be repaired.' really worries me, and I should be very grateful for advice.