If you can't delete a tag, and a duplicate is created when trying to add the tag to a new bookmark, it's likely that your places.sqlite file is corrupted. Tags are stored in the database as a special type of folder, and references to this tag may have been lost.

The easiest fix is to simply remove places.sqlite (which contains bookmarks and history), then import a recent backup of the bookmarks. This will repair any corruption and create a fresh database. Instructions for doing this are at Can't add, change or save bookmarks - How to fix

Do any bookmark show in the right pane in the Bookmarks Manager (Library) if you select those tags in the left pane?

See also Bookmark Tags - Categorize bookmarks to make them easy to find

Yes, they do show up in that column.

To delete a tag, you need to select all the bookmarks that show in the right pane if you select that tag in the left pane and use the context menu to delete them. That won't delete the bookmarks, but should remove the tag from those bookmarks. Once there are no bookmarks listed for a tag then that tag should disappear.