Nautilus fails to see shares in 18.04

There's two different issues in your description.

The "Windows Network" problem is due to samba changing a client protocol level. https://ubuntuforums.org/showthread.php?t=2384959

To restore it to how it used to work:

Install smbclient:

sudo apt install smbclient

Then edit /etc/samba/smb.conf and add a line under the workgroup = WORKGROUP line:

client max protocol = NT1

Save the file and reboot ... yes ... reboot.

Please read the forum post about why Samba changed this to see if it affects you.

The other issue about not finding 17.10 samba servers seems odd because starting with that release they should be "published" automatically regardless of protocol level. I would see if the following service is running on all your machines:

sudo service avahi-daemon status

If it's not running restart it:

sudo service avahi-daemon start

Changing the client max to NT1 should also fix the Ubuntu 17.10 discovery but the avahi method really should be working by default.