I am revising my answer to be a much better one.
The overall situation I'm experiencing is best described here:
(Although it didn't receive "an answer," it does link to other pages that do, and I've cited some of those pages here.)
The problem appears to be in the
vboxadd-guest-utils-service as discussed in this "askubuntu" page:
Virtualbox - folder does not automatically mount, Windows 7 host Ubuntu 17.10 guest
... which refers to this Ubuntu bug report:
At this writing, I'm trying to ferret out exactly why the service is failing at startup in my case, but I do observe that the service is not running, and that if on my system I execute the following command, the shares appear:
sudo systemctl restart vboxadd-service.service
Unlike the cited post, however, my status for the service shows several services, some "exited":
Loaded: loaded (/opt/VBoxGuestAdditions-5.2.18/init/vboxadd; enabled; vendor preset: enabled)
Active: active (exited) since Thu 2018-08-23 12:06:20 EDT; 5min ago
Process: 905 ExecStart=/opt/VBoxGuestAdditions-5.2.18/init/vboxadd start (code=exited, status=0/SUCCESS)
Main PID: 905 (code=exited, status=0/SUCCESS)
Aug 23 12:06:00 MyVM systemd: Starting vboxadd.service...
Aug 23 12:06:03 myVM vboxadd: VirtualBox Guest Additions: Starting.
Aug 23 12:06:20 MyVM systemd: Started vboxadd.service.
Loaded: loaded (/opt/VBoxGuestAdditions-5.2.18/init/vboxadd-service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2018-08-23 12:11:20 EDT; 3s ago
Process: 2696 ExecStart=/opt/VBoxGuestAdditions-5.2.18/init/vboxadd-service start (code=exited, status=0/SUCCESS)
Tasks: 8 (limit: 4665)
└─2737 /usr/sbin/VBoxService --pidfile /var/run/vboxadd-service.sh
Aug 23 12:11:19 MyVM systemd: Starting vboxadd-service.service...
Aug 23 12:11:20 MyVM vboxadd-service: vboxadd-service.sh: Starting VirtualBox Guest Addition service.
Aug 23 12:11:20 MyVM vboxadd-service.sh: Starting VirtualBox Guest Addition service.
Aug 23 12:11:20 MyVM vboxadd-service.sh: VirtualBox Guest Addition service started.
Aug 23 12:11:20 MyVM systemd: Started vboxadd-service.service.
● virtualbox.service - LSB: VirtualBox Linux kernel module
Loaded: loaded (/etc/init.d/virtualbox; generated)
Active: active (exited) since Thu 2018-08-23 12:06:08 EDT; 5min ago
Process: 1356 ExecStart=/etc/init.d/virtualbox start (code=exited, status=0/SUCCESS)
Aug 23 12:06:06 MyVM systemd: Starting LSB: VirtualBox Linux kernel module...
Aug 23 12:06:08 MyVM systemd: Started LSB: VirtualBox Linux kernel module.
Undoubtedly, this "confusion" has considerable bearing on my problem . . .