If worse come to worse, how can I get a list of all installed programs?
That way I can come close to restoring my system after mail-utils boned things up badly. 
I saw them in Package manager, but saw no way to select and copy them.
I have some clonezilla images, but they are about 10 days old.
AFAIK, you can use Synaptic to save a list of installed packaged. I never used the feature but I guess you have to select all the installed packages (that you can get from Status → Installed) and choose File → Save selections as. Then, later, File → Read selections to get them.
Be aware, though, that the packages don’t include your personal settings and data. if you plan to reinstall your OS from scratch and lose your home dir in the process, you won’t get your settings back by reinstalling the softwares. This includes stuff like the web browser’s bookmarks, e-mails in the mail client and so on.
I will be using a Clonezilla image.
I won’t lose too many customizations.
apt list --installed > Packages.txt
Yeah, that’s the command line equivalent of what Synaptic does (or is supposed to). Not sure you can easily use the list from APT to reinstall packages afterwards, though.
I take a screenshots of my desktop so I can know somewhat what I have installed.
Then I can use that apt list to re-install what I don’t have.
I hope someone can help me with my mail-utils issue so it doesn’t come to that.