How can I place a shortcut to a network folder in my sidebar?
Wow, it looks like I have found a workable solution to this.  Still a few strange bits which you can see in the new video I created, but the end result basically works.

Steps:


With the remote volume not mounted, open the Connect to Server dialog (Command-K in Finder)
Enter the network address of your remote volume.  If you have previously connected to it, it should be selectable in the dropdown menu on the right hand side.  The resulting address should look something like smb://Media%20Server._smb._tcp.local
Add the rest of the path to your desired folder, including the volume name.  In my case the volume was called Media1 and the folder was called backups, so the final result was smb://Media%20Server._smb._tcp.local/Media1/backups.  Click Connect.
Note that the resulting window displays the remote folder you wanted, but it is now mounted as if it were a volume rather than a folder.
Drag the icon in the titlebar of the window, to the sidebar.


Now you can mount and eject the remote folder directly from your sidebar.  There is still some strange behavior with occasional complaints when ejecting the volume, but they don't always happen and I'm wondering if they are caused by my being on Wifi on my Macbook.  Still in the end I seem to have a working solution after years of wrestling with this and never getting it to work reliably.