Hi good folks who all piled in with help. A quick update.
For the curious, I see traces in the metadata of Calibre having been involved in the conversion process. When I have tried Calibre for conversion (albeit some years back, before I discovered Sigil), I had to eliminate loads of useless spans but I never came across one like this, systematically cutting off the last 9 characters of every paragraph regardless of word breaks. I think it must be some idea configured into the process by my friend's book designer. I doubt it was aimed at widows and orphans, though, because these had specific parameters defined in the contents paragraphs' CSS in the stylesheet.
The good news is that with the Zoom call and a couple of emails, I managed to teach my author friend how to remove those spans with a regex. (including explaining what a regex is and does).
Her re-editing of her book is proceeding fine helped by occasional email exchanges and zoom calls. I doubt she will have any need for a book designer for her future books in her series once we get over the plugins hurdle so she can import her Word document into Sigil.
I use LibreOffice and the ODTimport plugin with my customised config.xml file which took me a while to sort out for style mapping. I suppose I'll have to go thro the whole thing again for a Word import plugin.
I'm going to add TagMechanic plugin to my Sigil and have a look at the tutorials mentioned above by @Tex2002ans.
Thanks again, everyone.