$sudo ls -la /var/lib/dpkg/
[sudo] password for frank:
total 10144
drwxr-xr-x 7 root root 4096 Jan 1 12:05 .
drwxr-xr-x 97 root root 4096 Nov 19 14:27 ..
drwxr-xr-x 2 root root 4096 Dec 31 13:07 alternatives
-rw-r--r-- 1 root root 11 Jul 1 2014 arch
-rw-r--r-- 1 root root 1356782 Jan 1 12:05 available
-rw-r--r-- 1 root root 1356782 Dec 31 13:07 available-old
-rw-r--r-- 1 root root 8 Jun 24 2014 cmethopt
-rw-r--r-- 1 root root 1092 Oct 21 13:22 diversions
-rw-r--r-- 1 root root 1136 Oct 21 13:21 diversions-old
drwxr-xr-x 2 root root 667648 Jan 1 12:05 info
-rw-r----- 1 root root 0 Jan 1 12:05 lock
drwxr-xr-x 2 root root 4096 Mar 7 2014 parts
-rw-r--r-- 1 root root 479 Nov 13 17:49 statoverride
-rw-r--r-- 1 root root 440 Nov 13 17:49 statoverride-old
-rw-r--r-- 1 root root 3469728 Jan 1 12:05 status
-rw-r--r-- 1 root root 3469764 Jan 1 12:05 status-old
drwxr-xr-x 2 root root 4096 Dec 25 12:48 triggers
drwxr-xr-x 2 root root 4096 Jan 1 12:05 updates

sudo ls -la /var/lib/apt/
total 184
drwxr-xr-x 6 root root 4096 Jan 1 12:05 .
drwxr-xr-x 97 root root 4096 Nov 19 14:27 ..
-rw-r--r-- 1 root root 185 Jul 1 2014 cdroms.list
-rw-r--r-- 1 root root 138243 Jan 1 12:05 extended_states
drwxr-xr-x 2 root root 4096 Jun 24 2014 keyrings
drwxr-xr-x 3 root root 20480 Jan 1 13:34 lists
drwxr-xr-x 3 root root 4096 Jun 24 2014 mirrors
drwxr-xr-x 2 root root 4096 Jul 4 2014 periodic

sudo ls -la /var/cache/apt/
total 95168
drwxr-xr-x 3 root root 4096 Jan 1 12:21 .
drwxr-xr-x 20 root root 4096 Apr 25 2016 ..
drwxr-xr-x 3 root root 90112 Jan 1 12:04 archives
-rw-r--r-- 1 root root 48793478 Jan 1 12:21 pkgcache.bin
-rw-r--r-- 1 root root 48580181 Jan 1 12:21 srcpkgcache.bin

sudo ls -la /etc/apt
total 120
drwxr-xr-x 6 root root 4096 Aug 17 16:06 .
drwxr-xr-x 186 root root 12288 Jan 1 12:05 ..
drwxr-xr-x 2 root root 4096 Jan 1 12:05 apt.conf.d
drwxr-xr-x 2 root root 4096 Jan 30 2017 preferences.d
drwxr-xr-x 2 root root 4096 Dec 25 12:48 sources.list.d
-rw------- 1 root root 40 Jun 24 2014 trustdb.gpg
-rw-r--r-- 1 root root 42052 Aug 6 12:45 trusted.gpg
-rw-r--r-- 1 root root 40425 May 8 2017 trusted.gpg~
drwxr-xr-x 2 root root 4096 Apr 10 2014 trusted.gpg.d

~$ sudo ls -la /etc/dpkg
total 36
drwxr-xr-x 4 root root 4096 Jun 27 2016 .
drwxr-xr-x 186 root root 12288 Jan 1 12:05 ..
-rw-r--r-- 1 root root 446 Jun 12 2013 dpkg.cfg
drwxr-xr-x 2 root root 4096 Jun 24 2014 dpkg.cfg.d
drwxr-xr-x 2 root root 4096 Aug 5 2016 origins
-rw-r--r-- 1 root root 260 Jun 12 2013 shlibs.default
-rw-r--r-- 1 root root 253 Jun 12 2013 shlibs.override

gksudo software-sources
Asks for password, then after inputting it -
Application().run()
File "/usr/lib/linuxmint/mintSources/mintSources.py", line 635, in __init__
if self.config["general"]["use_ppas"] == "false":
KeyError: 'general'

Thanks, starrfleat. I'm sorry for how long this is taking - thanks for sticking with me.

Those directory listings look fine at first glance. There are a couple of minor inconsistencies but only in comparison to my directories on Mint 18.3, so they might just be due to version differences.

The problem with mintSources.py looks crucial though. Can I ask where you obtained your original version of Mint? You didn't use a copy from Linux Format by any chance? The reason I ask is: https://forums.linuxmint.com/viewtop...f=228&t=167441

Just thanks for your patience in sticking with me! This is a real bug-a-bear!
I think I went through Distrowatch, and followed the posted link to http://www.linuxmint.com/download.php
I'm about tempted to throw in the towel and just install Mint 18.3 for gnome. I assume it's working for you?
Of course, that may not work. Dunno why this works on my other laptop, but not this one.
I wouldn't think having Virtualbox installed, even if just on this one, would make any difference. I haven't used it in a while.

Ok. That source should have been ok. At some point though the package manager has taken a hit, as shown by those faulty permissions we sorted out earlier on. I've no idea how.

We have two choices: carry on trying to resolve the problem (which might be good educationally but may not be successful), or reinstall. Mint 18.3 doesn't come with the Gnome desktop interface supported, it comes in Cinnamon, MATE, Xfce and KDE flavours (although this will be the last time Mint produces a KDE flavour). If you want to reinstall then I would recommend Cinnamon unless your machine has processor/memory constraints in which Xfce would probably be more suitable. I personally use MATE but just because I prefer its look.

A reinstall should work fine, but do ensure that you copy all your documents, images, videos, music and other such files to a backup device first.

No, having VirtualBox shouldn't make a difference. I use that on my machine with no problems.

Your call.

Well, let me sleep on it. I’ll try and have a decision by the afternoon EST tomorrow.

Finally installed Mint 18.3 cinnamon on my troubled laptop. Took excess time because it seem there was an additional prob which wouldn't let Unetbootin run to completion. After a new usb stick didn't help, finally ran it and the .iso on my spare laptop. Created the installation usb, and installed it.
Fruho now runs, and likely all else will. However, my hard drive now has two directories with my name: a small one within the Mint partition, and the old one on what I want to be my home partition. Guess I will have to look up how to fix that. Maybe have to just reinstall, and mark my separate home partition with that mount point.

OK, thanks for keeping us up-to-date. I'm glad that Mint 18.3 and Fruho are working for you. If you ever have problems again with Unetbootin, Mint has its own program mintstick, while Etcher is a great program for creating bootable USB sticks.

If you can't resolve that "two-directory" problem, please start a new thread on here, adding more details of what you're aiming for, what the problem is, and what the relevant choices are that you made during the installation procedure.

SOLVED
I reinstalled 18.3, setting the proper partition with mount point /home. Having to reconnect and reinstall some things, but all appears well. Had to add a line to fstab to initialize my windows partition for access at bootup.
For safety, I won't add new linux headers, and will wait for mint 19.3 before upgrading.Hopefully things will stay stable, thanks for staying with me while we worked this all out.

Just a couple of points. If by "upgrade" you mean the normal updating of the machine's software, I would advise against holding off on updating. Updates may contain security-related elements. What you should do is make a copy of your system (I use Macrium Reflect to image my partitions, but CloneZilla is more widely used and there are other options too). That way, you can more easily roll back if an update goes awry. In that regard, also have a look at the Mint-supported TimeShift which, if you install it and configure it, acts like Windows' Systems Restore.

If by "linux headers", you mean kernels, then what I would advise is that, unless there is a very pressing requirement to do otherwise, always update your kernels through the Update Manager. By the time that kernels reach the Ubuntu repos they have been tested fairly thoroughly. What I do personally is install any new kernels that Mint suggests and delete the oldest one at the same time, always keeping three kernels installed, the latest one of which is active. In that way, I can fall back to an older kernel if the latest one goes awry.

Finally, it's Mint 19 that will be up next, not Mint 19.3. We're probably talking June or thereabouts.

Sorry. I meant upgrading to the next version of Linux Mint. If I hold off until .3, I'm hoping to avoid the most programmatic "gotchas" .
I'll research Timeshift. Meanwhile, if I want to delete old headers, is Synaptic the way to go?

There's no reason to hold off until 19.3, honestly.

I never use Synaptic. I either use the Update Manager or the command line. Each kernel comes in a suite of packages. For example, the latest kernel on my system has:

If you do all your kernel additions/deletions in the Update Manager, it takes into account all of these separate packages. If you want to do things on the command line or in Synaptic, you have to be careful to deal with all the packages.