Wow
@Tex2002ans! I'm impressed by all the information on your post. Certainly I'm going to add TagMechanic

to my plugins in Sigil and let's see if future versions can advance in this area.
One of the things I would also love is if when Sigil can't save the book from HTML errors (missing < or > or a tag, etc) it would give you more information. The popup dialog doesn't identify the wrong file and right now I have to turn live preview and check all recently modified files to get the warning error in LP with the line number (when lucky).
Code View should mark the line with a red dot like you see in Text/Code Editors and switch to offending page/line as soon as you click on "Manually Correct"