Not sure it is Word that mangles the HTML as I do not save the Word text in anything other than it's standard doc format, I certainly do not save it in HTML. I believe it is the receiving editor that translates it from 'doc' format to HTML.
HTML appears, sorry is a very sloppy language that requires a bloated browser to read it, unlike the more rigorous XML and derivatives. This I beleive is where browsers, editors and previewers start interpreting the code in various ways and even show errors that do not exist when the listing goes live on eBay.
Summary, I don't know enough about HTML to make a clean job of it but copy/pastes from Word lets me do a lot more than I could otherwise. If I need to change the pasted text I edit the Word version and repaste. Works for me, and in IE6 with the design editor do not clean the paste !
