So at last it came down to this, the need to pick one or the other, but not 
both. I followed Slavek's recommendations, and managed to install packages 
that provided the systemd-type stuff that was needed for network-manager; 
thus I was able to install network-manager-tde, as well. 

But now I was bummed out, because I couldn't get gufw, couldn't have 
firestarter any more, couldn't have a gui firewall. I still had to figure out 
the inner workings of gufw, but it seemed my best hope of a firewall. 

Then it occurred to me ... I still have the packages for gufw, as well as the 
required dependencies ... evil thoughts were brewing ... time to work some 
forbidden self-hacking black magic! 

So I used dpkg and installed the firewall packages that I had already saved; 
to wit: 

firewalld 
gufw 
ifupdown 
policykit 
python3 
python3-gi 
python-ufw 
ufw

Mind you, ifupdown is a package for the network; but when I originally 
installed firewall packages, it somehow got caught up in the dependencies or 
recommendations. Then, in keeping with my habit, I moved the packages to a 
safe location: 

sudo mv -v -f /var/cache/apt/archives/*.deb -t /media/<my_safe_location>
and changed permissions in that folder, so that I could work with them, move 
them around to other folders, and so on (chown and chgrp). 

I started doing this because Internet was spotty, or because I worked in 
different locations a lot, or sometimes just because my networking was 
problematic, and packages can't be downloaded if you can't connect. But if 
you have them saved somewhere, no problem. (Yes, I know they say to prefer 
downloading from the repository, but you can always update later, once you 
actually get your system running!) 

And it all ends with everybody living happily ever after: Once I got not only 
tdenetworkmanager and network-manager installed, as well as gufw, I ran sudo 
apt-get -f install, just to see if it would try uninstalling something, and 
everything is just fine. 

So, if this helps anybody who wants to do the same, there you go. As I said, I 
would eventually get my system to do what I wanted. Now if we could make it 
so that the pseudo-conflict with systemd goes away (since it is only 
apparent, from what I can tell), that would be nice. But at least there is a 
possible way to work around it. 

Bill