From what I remember, XFCE likes network-manager-gnome package.  Install that, it installs ontop of NM, so leave him alone.  This *gnome package should provide an app called nm-applet.  It will be a panel/sys tray app. 

You may need to add the nm-applet to the panel/sys tray.  Someone else with XFCE installed should take over from here as I am running on faded memories.

As Micronaut says they prefer XFCE + selective gnome then, as you suggest, adding the relevant Gnome packages,until it works bits seems sensible. However interestingly the Devuan live CD  chooses to uses XFCE with wicd for network management. I had the live CD .iso working and then installed on my setup to prove wireless (and other services) worked OK under Devuan on my system but I then went back and used the installation DVD .iso + advanced install to configure with RAID1 (mirrored drives) and Cinnamon. Cinnamon gives me the same toolkit (such as Rhythmbox for sound, Evolution for mail, Network Manager for WiFi) and interface as I had  when I was using Mint (and before that Gnome 2 Ubuntu).

You may need to add the nm-applet to the panel/sys tray.  Someone else with XFCE installed should take over from here as I am running on faded memories.

After installing the network-manager package I have not found any graphical network management utilities of any sort. Not in the menus, and not in the list of available objects when adding things to the XFCE toolbar. Where would I look?

/etc/network/interfaces should be this:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

Question -- where do you find the controls/config for NetworkManager? It doesn't have any entries in the System or Settings menus.

To be more specific, they are generally in the panel/system tray app.  After installing NM, you may need to add the network panel app to the panel/system tray.  This all depends on what desktop env you are using.  What are you using; XFCE, LXQT, KDE, MATE....?

Of course, that may not fix your boot problem as you say it may be an specific issue with your motherboard or its twin ethernet ports: mine's a Gigabyte board of 2011 vintage.

As @panopticon mentions, there is the nice nmtui console interface.  There is also the nmcli commandline interface.

The config connections for NM are in /etc/NetworkManager/system-connections/.  But the panel apps provide full config in themselves.

I thought wicd was for Wifi only, but I've also been struggling with a laptop and when I set wicd up, it does watch both the cable connection and the wifi. And it puts a widget on the task bar! At last resort, I may just use wicd even on my desktop systems. I don't plan to put wifi on them, but oh well... it manages direct cable connections, too.

I'm aware of mntui, used it several times even, but it's still a graphical front-end (be it console-based). Its man-page is rather limited in informational value though