I checked case (B) that I added.
First I created a new setup as described (to make sure no old modifications influence the verification)
That worked, I saw the wait line on boot:
[***   ] A start job is running for Wait for… to be Configured (29s / no limit)

After boot I had the wait time in the failed service:
  $ systemctl status systemd-networkd-wait-online
● systemd-networkd-wait-online.service - Wait for Network to be Configured
   Loaded: loaded (/lib/systemd/system/systemd-networkd-wait-online.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Tue 2019-04-16 13:52:04 UTC; 15s ago


Installing the version in proposed ...
$ sudo apt install systemd=237-3ubuntu10.21
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  grub-pc-bin
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  libnss-systemd libpam-systemd libsystemd0
Suggested packages:
  systemd-container
The following packages will be upgraded:
  libnss-systemd libpam-systemd libsystemd0 systemd
4 upgraded, 0 newly installed, 0 to remove and 42 not upgraded.
Need to get 3318 kB of archives.
After this operation, 11.3 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 libnss-systemd amd64 237-3ubuntu10.21 [105 kB]
Get:2 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 libpam-systemd amd64 237-3ubuntu10.21 [108 kB]
Get:3 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 systemd amd64 237-3ubuntu10.21 [2901 kB]
Get:4 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 libsystemd0 amd64 237-3ubuntu10.21 [204 kB]
Fetched 3318 kB in 1s (4658 kB/s)
(Reading database ... 60080 files and directories currently installed.)
Preparing to unpack .../libnss-systemd_237-3ubuntu10.21_amd64.deb ...
Unpacking libnss-systemd:amd64 (237-3ubuntu10.21) over (237-3ubuntu10.19) ...
Preparing to unpack .../libpam-systemd_237-3ubuntu10.21_amd64.deb ...
Unpacking libpam-systemd:amd64 (237-3ubuntu10.21) over (237-3ubuntu10.19) ...
Preparing to unpack .../systemd_237-3ubuntu10.21_amd64.deb ...
Unpacking systemd (237-3ubuntu10.21) over (237-3ubuntu10.19) ...
Preparing to unpack .../libsystemd0_237-3ubuntu10.21_amd64.deb ...
Unpacking libsystemd0:amd64 (237-3ubuntu10.21) over (237-3ubuntu10.19) ...
Setting up libsystemd0:amd64 (237-3ubuntu10.21) ...
Processing triggers for ureadahead (0.100.0-20) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Setting up systemd (237-3ubuntu10.21) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for dbus (1.12.2-1ubuntu1) ...
Setting up libnss-systemd:amd64 (237-3ubuntu10.21) ...
Setting up libpam-systemd:amd64 (237-3ubuntu10.21) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...


Rebooting:
- bootup was fast
- service works fine
  $ systemctl status systemd-networkd-wait-online
● systemd-networkd-wait-online.service - Wait for Network to be Configured
   Loaded: loaded (/lib/systemd/system/systemd-networkd-wait-online.service; enabled; vendor preset: enabled)
   Active: active (exited) since Tue 2019-04-16 13:54:20 UTC; 10s ago


Thereby good for case B, once A is confirmed as well please set it to verified.