If you have MS Word 2007 or later you can use
Toxaris's eBook add-in for Word to create ePub 2 files with nothing in between.
Calibre always flattens the CSS, including those from WP styles in DOCXes.
The Mammoth tool (the Sigil plugin is a Mammoth wrapper) requires mapping of DOCX styles to CSS entries, which means inheritance (cascade), and style names can be preserved. If you don't provide the mapping you'll lose most of the styling - which can be useful at times.
BR